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THE INFLUENCE OF MICROFOSSIL CONTENT

ON THE PHYSICAL PROPERTIES OF CALCAREOUS SEDIMENTS

FROM THE ONTONG JAVA PLATEAU

A DISSERTATION SUBMITTED TO THE GRADUATE DIVISION OF THEUNIVERSITY OF HAWAI'I IN PARTIAL FULFILLMENT OF THE

REQUIREMENTS FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

IN

GEOLOGY AND GEOPHYSICS

MAY 1995

By

Janice Christine Marsters

Dissertation Committee:

Murli Manghnani, ChairpersonJames Cowen

Loren KroenkeJohanna Resig

Jane TribbleRoy Wilkens

UMI Number: 9532607

OMI Microform 9532607Copyright 1995, by OMI Company. All rights reserved.

This microform edition is protected against unauthorizedcopying under Title 17, United States Code.

UMI300 North Zeeb RoadAnn Arbor, MI 48103

ACKNOWLEDGEMENTS

In geology, a "superindividual" is defined as an aggregate of grains that

behaves as a unit in the fabric of a rock. I have been truly blessed by "super

individuals" among my family, friends and colleagues who, together, have

made this process possible and endurable. I now find myself faced with the

daunting task of acknowledging their gifts in a few words.

I thank my parents for their gifts of love and unconditional support.

They may not have understood some of the choices I made, but they have

always encouraged me (in words and through their own example) to do the

best I could in whatever the task. My father's innate common sense, his

ability to do anything he sets his mind and hands to, and his love of math

and science were wonderful to experience as a child, and have fostered in me

a deep reverence for discovery. My mother's persistent and thoughtful love

has been a steadying force all my life. I thank my siblings Dean, Stephen and

Dawn for all their phone calls. Our mutual love of geology makes us an odd

family! I thank my grandparents for their quiet faith in me, and especially

grandmother Anne MacDonald for all her letters that have kept me in touch

with my far-away home.

I thank my teachers for their gifts of knowledge and empowering

confidence in my abilities. I was blessed to have attended a small school

where female students were never discouraged from math and science.

I have a fond place in my heart for the late Allister Clark, my high school

principal and teacher who, perhaps partly because he had two daughters for

whom he wanted the world, challenged his female students to be the best

they could be. I thank Dr. Leslie Baikie, my master's degree chairperson at the

iii

Technical University of Nova Scotia, for suggesting I take on a project with

the marine geotechnical group at the Atlantic Geoscience Centre (Bedford

institute of Oceanography), thus spawning my love of marine geology.

I thank my colleagues in Canada for their gifts of mentorship and

friendship, and for showing by example what it is to love science (and also

how to survive those long days at sea on a research vessel). I especially thank

Kate Moran for sharing her joy of science, strength, and camaraderie (and

some good aerobics moves). I hope we solve puzzles together for a long time

to come! I thank Kate Jarrett for her assistance and laughter during testing in

the geotechnical lab at the Atlantic Geoscience Centre and Patricia Stoffyn for

assistance with SEM analyses. I thank David Mosher for his friendship and

off-the-wall humor during many cruises. I especially thank Larry Mayer for

suggesting that I come to Hawai'i and for caring how I did here.

I thank my colleagues at the University of Hawai'i for their gifts of

encouragement and acts of kindness. I thank Neil Fraser for his confidence in

me and for not letting me give up. I thank Alexander Shor for his support in

many guises. I thank my Ph.D. committee members, James Cowen, Loren

Kroenke, Johanna Resig, Jane Tribble, Roy Wilkens, and especially my

chairperson, Murli Manghnani, for their guidance, encouragement, and

constructive criticism. I thank Fred MacKenzie for his guidance as an early

member of my committee. I thank Don McGee for his assistance with SEM

analyses, John Balogh for assistance in the mineral physics laboratory, and

Johanna Resig and James Wilcoxon for the microfossil analyses used in this

dissertation.

I thank Loren Kroenke, Wolfgang Berger, and the scientific party of

ODP Leg 130 for their strong support of the physical properties program. I also

iv

thank the shipboard technical staff of the Ocean Drilling Program for their

efforts in obtaining good-quality samples. I am thankful for the opportunities

I have had through public funding: participation on Ocean Drilling Program

Leg 130 and post-cruise research, including consolidation testing and

microfossil analyses, were supported by USSAC funding.

I thank my coworkers and friends at Masa Fujioka & Associates,

particularly Masa Fujioka and Jennifer Kleveno, for their gifts of support

during this process. I know they lightened my work load considerably (by

taking more work upon themselves) during the last few months. Their

encouragement and confidence in my abilities made this a much easier task.

I thank my friends for their gifts of joy, kindness, and love. I thank my

friends at home in Canada, J. J. Jansen, Leah Clark, Laurel Clark, Anne

Ackerman, and Nancy Dyke for all their good thoughts and kind messages. I

am blessed to have had many good friends in Hawai'i whose kind thoughts

and wishes helped me through this task. In particular, I thank Amy Sheridan

and Eric Halter for their thoughtful caring. I thank Robin Brandt for dancing

hula with me in our living room when I couldn't think about science

anymore and for feeding me tea and scones. I thank Leolani Abdul for her

good humor and the sweetened condensed milk. I thank Johanna Resig for

art-full evenings and quietly reminding me I still had this to finish. And I

especially thank Michael Tanenbaum for his patient love and support, and

for being there when I needed him.

Finally, I thank my friends and fellow geology graduate students for

their gifts of insight and compassion and for sharing parts of the journey with

me. In particular, I thank Beth Jorgenson for sharing lots of guffaws and for

telling it like it is. I thank Frank Trusdell for many moments of joy and

v

laughter and for instigating my thoughts of Hawai'i as my home. I thank

Suki Smaglik for her many selfless acts of kindness and for sharing her love

of Hawai'i. And I especially thank Mary MacKay for her unwavering

encouragement, for always listening, and for dinner and companionship

during some of my late nights at the computer.

For me, the blessing has not been the completion of this work. It has

been having these cherished people, the "super individuals" of my life, share

the journey with me. Mahalo nui loa.

vi

ABSTRACT

This work is based on results from Ocean Drilling Program Leg 130,

which drilled the Ontong Java Plateau, a broad submarine plateau in the

western equatorial Pacific. The Ontong Java Plateau has a unique

combination of geographic location and bathymetry that makes it ideally

suited for paleoceanographic studies. This study addresses several aspects of

plateau sediment physical properties (e.g., porosity, velocity, acoustic

impedance, compression and expansion indexes, and rebound) within the

broad framework of establishing relationships between physical properties

and microfossil content and preservation.

Samples obtained in Hole 803D were analyzed to determine their

microfossil constituents. The resulting data are compared to shipboard­

measured physical properties data to assess the relationships between small­

scale fluctuations in physical properties and microfossil content. Impedance

was found to increase with increasing grain size and planktonic foraminifera

content. Variations in the coarse fraction constituents appear to have a more

significant effect on the physical properties than do variations in the fine

fraction constituents, though the fine fraction make up greater than 85% of

the samples by weight. Many of the seismic reflectors identified by shipboard

scientists could be related to changes in the relative percentages of microfossil

constituents.

The consolidation behavior of Ontong Java Plateau sediments was

observed to relate to sediment composition. In general, consolidation test

parameters from this study are consistent with those of other researchers for

sediments of similar carbonate contents. We found that both compression

vii

and expansion indices decrease with increasing carbonate content and with

increasing foraminifer content.

Rebound curves derived from consolidation tests on Ontong Java

Plateau samples yield porosity rebounds of 1% to 4% for these sediments at

equivalent depths of 200 to 1200 meters below seafloor (mbsf), The exception

is a radiolarian-rich sample that has 6% rebound. We combined the rebound

correction derived from the porosity rebound vs. depth data with the

correction for pore-water expansion to correct the shipboard laboratory

porosity data to in-situ values. The rebound-corrected laboratory data can be

used as in situ data in place of missing or erroneous downhole logging data.

viii

TABLE OFCONTENTS

ACKNOWLEDGEMENTS iii

ABSTRACT ,......... vii

LIST OF TABLES xi

LIST OF FIGURES xii

CHAPTER 1: INTRODUCTION 1

CHAPTER 2: RELATIONSHIPS BETWEEN PHYSICAL PROPERTIESAND MICROFOSSIL CONTENT AND PRESERVATION. 14

Background 14

Objectives of this Study............................................................... 15

Experimental Procedures 16

Shipboard Physical Properties Analyses 16

Microfossil Analyses 18

Results 20

Physical Properties 20

Microfossil Content and Preservation 22

Correction of Downhole Profiles 24

Discussion 26

Role of Intraparticle Porosity........................................... 26

Influence of Cementation 29

Relative Significance of Fine Fraction Constituents.. 31

Relationships with Other Shipboard Data 31

Effect of Post-Burial Dissolution 32

Seismic Reflectors .. 33

Conclusions 37

ix

CHAPTER 3: INFLUENCE OF MICROFOSSIL CONTENT ONCONSOUDATION PROPERTIES 71

Introduction 71

Consolidation Theory 72

Procedures 76

Consolidation Tests 76

Other Analyses 79

Results 80

Discussion 82

Consolidation Test Data for Carbonates 82

Pc' and Stress History.................... 83

Shape of Consolidation Curves and IntraparticleWater 84

Correlations with Microfossil Content 86

Conclusions 87

CHAPTER 4: POROSITY REBOUND CONTENT OFONTONG JAVA PLATEAU SEDIMENTS 111

Introduction 111

Shipboard Procedures 112

Rebound Model from Consolidation Data 113

Correction of Shipboard Porosity Data 115

Conclusions 119

CHAPTER 5: SUM!vIARY . 135

REFERENCES 138

x

Table 2-1

Table 2-2

Table 3-1

Table 3-2

LIST OF TABLES

Coarse fraction constituents for Hole 803D 39

Fine fraction constituents for Hole 803D 42

Consolidation test results 89

Microfossil analyses of four consolidation samples 90

xi

LIST OF FIGURES

Figure 2-1 Synthetic seismogram and field seismic record forSite 803 46

Figure 2-2 Downhole profiles of porosity, velocity, and calciumcarbonate for the upper 300 m of Hole 803D 48

Figure 2-3 Results of microfossil analyses; (a) grain size vs. depth,(b) coarse fraction, and (c) fine fraction components 50

Figure 2-4 Correction of downhole porosity and velocity data 52

Figure 2-5 Correction of downhole microfossil data 54

Figure 2-6 Total porosity vs. percentage of coarse fraction andthree coarse fraction constituents .. 56

Figure 2-7 Total porosity of foraminiferal assemblages andglass beads 58

Figure 2-8 Interparticle porosity vs. percentage of coarse fraction andthree coarse fraction constituents 60

Figure 2-9 Velocity vs. percentage of coarse fraction and threecoarse fraction constituents 62

Figure 2-10 Velocity vs. percentage of coarse fraction and threecoarse fraction constituents for sample depths <150 mbsf .... 64

Figure 2-11 Impedance vs. percentage of coarse fraction and threecoarse fraction constituents 66

Figure 2-12 Total porosity, interparticle porosity, velocity, andimpedance vs. percentage of nannofossils 68

Figure 2-13 Seismic reflectors on downhole profiles of impedance,calcium carbonate, and microfossil constituents 70

Figure 3-1 Calcium carbonate data for five sites 92

Figure 3-2 Schematic showing relative water depths of five sites 94

xii

Figure 3-3 Void ratio vs. effective stress for samples from Site 803 96

Figure 3-4 Void ratio vs. effective stress for samples from Site 804 98

Figure 3-5 Void ratio vs. effective stress for samples from Site 805 100

Figure 3-6 Void ratio vs. effective stress for samples from Site 806 102

Figure 3-7 Void ratio vs. effective stress for samples from Site 807 104

Figure 3-8 SEM images of consolidation samples 106

Figure 3-9 Compression index and expansion index vs.carbonate content 108

Figure 3-10 Compression index and expansion index vs.foraminifer content 110

Figure 4-1 Merged porosity vs. depth for Site 806 and generalizedlaboratory curve for calcareous sediments 122

Figure 4-2 Hole 8030 downhole porosity profiles 124

Figure 4-3 Rebound in % porosity change vs. pressure 126

Figure 4-4 Rebound in % porosity change vs. depth below seafloor ..... 128

Figure 4-5 Hole 8030 porosity data corrected for rebound and forseawater density.................................... 130

Figure 4-6 Hole 8030 downhole porosity corrected using combinedrebound and seawater-density correction 132

Figure 4-7 Hole 806B downhole porosity corrected using combinedrebound and seawater-density correction 134

xiii

CHAPTERl

INTRODUCTION

Overview

This work is based on results from Ocean Drilling Program (ODP) Leg

130, which drilled the Ontong Java Plateau (OJP), a broad submarine plateau

in the western equatorial Pacific. Specifically, this work examines the

influence of microfossil content on the physical properties of calcareous

sediments from the plateau.

Geologic Setting

The Ontong Java Plateau is the largest oceanic flood basalt plateau in

the world (Figure 1-1), with an approximate area of 1.5 million km2

(Mahoney, 1987). Based on drilling results on the Ontong Java Plateau as

well as field geological sampling on the island of Malaita, the Ontong Java

Plateau apparently began to form prior to 120 Ma (Tarduno et al., 1991;

Mahoney et al., 1993a; Mahoney et al., 1993b), probably along a west­

northwest-aligned spreading ridge. Pelagic sediments were deposited on the

plateau, and a shift from Austral to Tethyan assemblages at about 100 Ma

reflects the northward movement of the plateau.

In late Oligocene time, the southwestern part of the plateau apparently

encountered the Outer Melanesian (North Solomon) subduction zone,

ending subduction of the Pacific plate beneath the Outer Melanesian Arc

(Kroenke et al., 1986; Yan and Kroenke, 1993). Subduction ceased in the early

Miocene (about 25 Ma) when the convergent boundary shifted elsewhere,

1

with subduction resuming south of the Solomon Islands region in the late

Miocene (about 10 Ma), forming the New Britain-San Cristobal Trench.

Eastward subduction of the Indo-Australia plate beneath the Pacific

plate brought about the subsequent collision of the Woodlark Spreading

Ridge with the Solomon Islands Arc (about 4 Ma), leading to the elevation

and folding of the southwestern margin of the Ontong Java Plateau (Kroenke

et al., 1986; Resig et al., 1986). The Malaita Anticlinorium was formed in

conjunction with the overthrusting of the Solomon Islands Arc by plateau

oceanic crust.

Paleoceanographic Objectives of PDP Leg 130

Previous investigations in the central equatorial Pacific identified a

series of seismic reflection horizons synchronous over a large area of the

central equatorial Pacific seafloor. These reflection horizons appear to

correlate with major reorganizations of the oceanic circulation system (e.g.,

the initiation of northern hemisphere glaciation, the closing of the Tethys,

ice buildup in Antarctica, the opening of the Drake Passage) that are the

result of tectonic, climatic, and oceanographic processes (Mayer et al., 1985,

1986). The specific response of the equatorial sediment system to these major

paleoceanographic reorganizations is the increased dissolution of calcium

carbonate, and the impedance contrasts caused by these major dissolution

events result in seismic horizons.

While these events are useful from a seismic or stratigraphic

perspective, the complete removal or severe condensation of the section that

results from such dissolution over most areas of the central Equatorial Pacific

2

makes the detailed evaluation of paleoceanographic indicators at these

critical times virtually impossible. Therefore, although there is clear

evidence for a series of major paleoceanographic events, their expression as

dissolution events precludes the examination of many of the key parameters,

e.g., isotopes, faunal changes, chemical tracers, etc., that can be used to

characterize the paleoceanographic change.

The Ontong Java Plateau, however, has a unique combination of

geographic location and bathymetry that makes it ideally suited for detailed

paleoceanographic studies. Firstly, the plateau is presently, and has been for

a good part of its history, located near the equatorial zone of high production

of biogenous sediments. More importantly, the surface of the plateau has

stood above the carbonate compensation depth (CCD) throughout most of its

history. This combination of high production and bathymetry has resulted

in the accumulation and preservation of a thick (approximately 1 km)

sequence of pelagic carbonate sediments of Mesozoic and Cenozoic age that

has not been subjected to pervasive dissolution. The bathymetric

relationships extant today appear to have remained constant throughout the

history of the plateau (Resig et al., 1976). Although there is considerable

evidence for disturbance, and even mass wasting, of plateau sediments along

the margins of the Ontong Java Plateau (Berger and Johnson, 1976), much of

the central plateau exhibits virtually undisturbed sections, indicated by a

layer-cake stratigraphy (Mayer et al., 1991).

One of the objectives of ODP Leg 130 was to drill a series of equatorial

sites running from the top of the plateau to near its base, traversing nearly

2000 m of depth range in a relatively small geographic area, and sampling

3

sediments exposed to both deep and bottom water masses (upper and lower

deep waters). The sediments sampled would have been produced under

nearly the same surface-water conditions and thus in the same pelagic rain.

Sediments sampled along this transect would not be subject to many of the

variables normally associated with pelagic sedimentation (i.e., productivity

and latitudinal gradients) and would provide an ideal opportunity to

evaluate the vertical distribution of a range of parameters.

Four of the five sites drilled during ODP Leg 130 (Sites 803, 804,805,

and 806) form a depth transect down the northeast flank of the plateau

(Figure 1-2), bracketing a depth interval of 2500-3900 m. Within this depth

range, changes in dissolution gradients are most pronounced (Berger and

Johnson, 1976; Berger and Mayer, 1978). These dissolution gradients

correspond to differences between sites in microfossil content and

preservation, with considerable effects on physical properties and seismic

reflectors (Berger and Johnson, 1976; Berger and Mayer, 1978). A schematic of

the northeast flank of the Ontong Java Plateau, showing the relative

locations of drill sites in the depth transect and distinct seismic reflectors

(labeled A through E), is presented as Figure 1-3.

physical Properties and Paleoceanographic Events

An important key in the interpretation of seismic reflectors and other

changes in physical properties as paleoceanographic events is an

understanding of the relationships between dissolution events and physical

properties. Previous studies have suggested a link between the measured

4

physical properties of calcareous sediments and microfossil content and

preservation.

Johnson et al. (1977) found that the mean grain size of carbonates

decreases at deeper depositional sites on the periphery of the Ontong Java

Plateau, primarily because of the increased dissolution of foraminifera at

greater water depths. They found that deeper water sediments have lower

sound velocities and lower shear strength. Mayer (1979) studied eastern

equatorial Pacific sediments and found that low carbonate contents

corresponded to low densities, which he attributed to the relatively low

density of silica, which made up a significant portion of the sediments with

low carbonate contents.

This dissertation addresses several aspects of sediment physical

properties (e.g., porosity, velocity, acoustic impedance, compression index,

and rebound) within the broad framework of establishing relationships

between physical properties and microfossil content and preservation.

Earlier versions of two of the chapters have been previously published, as

detailed below. Previously published material has been reorganized to

provide a common abstract, introduction, and reference list for this

dissertation to reduce repetition. Some additional material has also been

included that was not part of the previously-published papers.

Chapter 2, entitled "Relationships between physical properties and

microfossil content and preservation," examines these relationships for

calcareous ooze samples from Site 803. Downhole profiles of porosity and

velocity show general trends with depth. Imprinted on these trends are a

number of small-scale fluctuations, with depth frequencies on the order of

5

meters or tens of meters. This study was initiated to determine if a

measurable link between sediment composition and measured physical and

acoustical properties could be determined. The samples from Hole 803D

obtained for the shipboard determination of index properties were retained

for the shore-based analysis of microfossil constituents.

Chapter 2, entitled "Relationships between physical properties and

microfossil content and preservation," was previously published as a 1993

paper of the same name by J. C. Marsters, J. M. Resig, and J. A. Wilcoxen

(Proceedings of the Ocean Drilling Program, Scientific Results, p. 641-652).

J. Marsters performed the physical properties measurements, analyzed the

data, and was the primary author of this previously-published manuscript.

Chapter 3, entitled "Influence of Microfossil Content on Consolidation

Properties of Ontong Java Plateau sediments", examines the consolidation

behavior of Ontong Java Plateau sediment samples from the five sites drilled

during ODP Leg 130. The effects of microfossil content and preservation, in

particular the relative percentage and preservation of whole foraminifera, on

consolidation behavior is discussed. Some of these consolidation data were

previously published in a 1993 paper entitled "Consolidation Test Results

and Porosity Rebound of Ontong Java Plateau Sediments," by J. C. Marsters

and M. H. Manghnani (Proceedings of the Ocean Drilling Program, Scientific

Results, p. 687-693).

Chapter 4, entitled "Porosity Rebound of Ontong Java Plateau

Sediments," uses rebound data from the nineteen consolidation tests to

correct shipboard physical property laboratory data to approximate in-situ

conditions. The effects of microfossil content and preservation on rebound

6

behavior is discussed. An earlier version of this chapter was included in the

Marsters and Manghnani (1993) publication cited above. J. Marsters analyzed

the data and was the primary author of the previously-published

manuscript.

Finally, Chapter 5 provides a summary of this work and discusses its

relationship to other recent studies of Ontong Java Plateau sediments and

calcareous sediments elsewhere. Directions for future research are also

suggested.

7

Figure 1-1. Location of the Ontong Java Plateau in the western equatorial

Pacific, showing the placement of drilling sites (after Kroenke et

al., 1983; Mammerickx, 1984). Box shows Leg 130 sites. Contour

interval is 500 m.

8

9

Figure 1-2. Bathymetry, in meters, of the northwestern part of the Ontong

Java Plateau (after Kroenke et al., 1983; Mammerickx, 1984). The

locations of the Leg 130 sites (Sites 803-807) together with

previous DSDP drilling sites are shown.

10

r 1600

11

Figure 1-3. Simplified acoustic stratigraphy for the flank of the Ontong Java

Plateau (Kroenke et al., 1991), and approximate locations of Sites

803 to 806 (the paleoceanographic depth transect).

12

......C>J

3

QlE~ 4

~in'

..c ~iiiQ) 0O~

'0UI

-g 58Q)

~

6

wswSites 269/586

..Acoustic basement ­

Sub-basement reflectors

OntongJava+- Plateau

o

Kilometers

100 200

Nauru Basin

ENE

"-~

CHAPTER 2

RELATIONSHIPS BETWEEN PHYSICAL PROPERTIESAND MICROFOSSIL CONTENT AND PRESERVATION

Background

During ODP Leg 130, shipboard scientists (Kroenke et al., 1991)

generated synthetic seismograms at each drill site using corrected laboratory

velocity and density data merged with the downhole log velocity and density

data. These data were used to calculate acoustic impedance (the product of

velocity and density) and reflection coefficients (the rate of change of the

acoustic impedance). The reflection coefficient profile was convolved with

the water-gun seismic source signature measured on the site survey cruise to

generate a synthetic seismogram for each site. The model is further described

in Kroenke et al. (1991) and Mayer et al. (1985).

A comparison of the synthetic seismogram (filtered with the same

filter parameters as the field record) with the field seismic profile collected at

Site 803 (Shipboard Scientific Party, 1991a) reveals an excellent match

between the two (Figure 2-1). Significant reflectors are clearly identifiable on

both the synthetic seismogram and the field record. Shipboard scientists

concluded that this correlation between the field seismic record and the

synthetic seismogram indicated that the traveltime-to-depth conversion was

fairly accurate (close examination reveals that the model-determined

velocities are about 1.2% too high). Given an acceptable traveltime-to-depth

conversion, shipboard scientists attempted to explain the origin of some of

the reflectors at Site 803 based on preliminary shipboard data.

14

Shipboard scientists (Shipboard Scientific Party, 1991a) identified 11

major reflectors or reflector packages (labeled 3-1 through 3-11) on the field

record at Site 803 (Figure 2-1). The selection of these reflectors was based on

their amplitude and lateral coherency within the immediate area of Site 803;

no effort was made to select regionally correlatable reflectors. In addition,

three reflectors (3a-3c) that are large-amplitude events on the synthetic record

but are less well-defined on the field record (at least at the exact location of

Site 803) were identified. Reflectors were not picked in the upper 30 ms of

the seismic record because of the disrupting effect of the outgoing pulse.

In most cases, a major reflector could be associated with a change in

the merged laboratory/downhole log acoustic impedance (Shipboard

Scientific Party, 1991a). Shipboard scientists (Shipboard Scientific Party,

1991a) used Site 803 preliminary shipboard physical properties (i.e., density,

velocity, and acoustic impedance) , lithologic (i.e., grain size and carbonate),

and biostratigraphic (Le., microfossil content) data to speculate on the cause

of the reflectors.

Objectives of this Study

This study was undertaken to determine if downhole fluctuations in

physical properties could be directly related to variations in microfossil

content and preservation and, subsequently, paleoceanographic events. We

wanted to (1) examine empirical relationships between physical properties

and microfossil content; and (2) examine the speculated causes of the

previously discussed seismic reflectors by analyzing variations in microfossil

content at reflector depths.

15

To accomplish these objectives, the samples used for the shipboard

determination of index properties (i.e., bulk density, porosity, water content)

and carbonate content of Hole 803D were retained for shore-based analyses of

their microfossil constituents. The depth intervals of these samples also

matched those of the shipboard velocity measurements. We anticipated that

the use of the same samples for the analyses of both microfossil and physical

properties parameters would provide a direct link between sediment

composition and measured physical and acoustical properties. The study was

necessarily restricted to the sediment interval that could be readily

disaggregated for microfossil analyses (the upper 270 m of this hole).

Experimental Procedures

Shipboard Physical Properties Analyses

The physical properties (velocity and index properties) data used in

this study were obtained as part of the shipboard physical properties program.

Compressional wave velocity was measured using a Dalhousie

University/Bedford Institute of Oceanography Digital Sound Velocimeter

(D5V; Mayer et al., 1987). One or two velocity measurements were

performed per 1.5-meter section of core. Velocity calculations were based on

the accurate measurement of the time of flight of an impulsive acoustic

signal traveling between a pair of piezoelectric transducers inserted in the

split sediment cores. The signal used was a 2-second square wave; the

transducers have resonances at about 250 and 750 kHz.

The transmitted and received signals were digitized by a Nicolet 320

digital oscilloscope and transferred to a dedicated microcomputer for

16

processing. The DSV software selected the first arrival and calculated

compressional-wave velocity through the sediment. Thermistors in the

transducer probes monitored temperatures while measurements were being

performed. The velocity data are corrected to a temperature of 25°C. Further

details of the equipment and methods can be found in Shipboard Scientific

Party (1991b).

Compressional-wave velocity was measured along both the vertical

(perpendicular to bedding) and horizontal (parallel to bedding) directions. In

general, the two velocity values at each sample interval are very close for the

soft oozes of this study; an average of the two values is used here.

For the shipboard measurement of index properties (density, porosity

and water content), samples were placed in preweighed aluminum

containers, and the weights determined to a precision of ±O.01 g, using a

motion-compensating Scitech electronic balance. The samples were dried in

a 110°C oven for 24 hours, cooled in a desiccator, and then weighed again to

obtain their dry weights. Wet and dry volumes of the samples were

determined to an approximate precision of 10-4 cm3 using a Quantachrome

helium-displacement pycnometer. Further details of the procedures can be

found in Shipboard Scientific Party (1991b).

Methods used for calculation of index properties are described in

Shipboard Scientific Party (1991b), and follow the methods of Boyce (1976).

Porosity was calculated as the ratio of the volume of voids to the total sample

volume, and is expressed as a percentage of the total sample volume.

Therefore, porosity represents the fraction of sample volume that is occupied

by pore space. Bulk density as the ratio of the total sample mass to total

17

sample volume. Index property data were corrected for salt content

according to the equations of Noorany (1984).

Shipboard determinations of carbonate content were performed using

a carbon dioxide colometer equipped with a carbonate carbon analyzer. Dried

samples of known mass were reacted in a 3N HCl solution. The liberated

C02 was titrated in a monoethanolamine solution with a colorimetric

indicator, and the change in light transmittance was monitored with a

photodetection cell. The percentage of carbonate was calculated from the

inorganic carbon content assuming that all carbonate occurs as calcium

carbonate. Further details of procedures and calculations are found in

Shipboard Scientific Party (1991b).

Shipboard bulk grain size determinations were conducted on

approximately one sample per section on unconsolidated cores. The depth.

intervals of these samples did not necessarily correspond with the depth

intervals of the physical properties/microfossil samples. Grain size analyses

were conducted using an eight-channel particle-size analyzer, with data

collected at the following size intervals: <4,4-8,8-16, 16-32,32-63, 63-125, 125­

250, and >250 !lID. Standard dispersal techniques were used to disaggregate

the samples for analysis. Further details of procedures and calculations are

described in Shipboard Scientific Party (1991b).

Microfossil Analyses

We retained the samples used in the shipboard index properties

determinations for the shore-based microfossil analyses. The dry samples

were weighed, soaked in water, and then wet sieved through a screen with

18

63-~ openings to isolate the coarse (sand) fraction. The combined fine (silt

and clay) fraction was washed into a bucket. After drying, the weight of the

coarse fraction was recorded for later calculation of the weight percent coarse

and fine fractions.

The dry coarse fraction was split by microsplitter to about 1/128 to 1/256

of its original size. The particles in the split sample, which numbered about

1000, were identified and counted. Microfossils ~ 50% intact were counted as

whole specimens. Principal components of the sand fraction were

planktonic foraminifers, fragments of planktonic foraminifers, and

radiolarians, which fluctuate in their relative frequency (Table 2-1).

Light microscope slides were prepared for the study of the fine

fraction. Approximately 1/2 gram of the fine-fraction sample was stirred into

a 10-ml beaker of water. A few drops of the resultant mixture were removed

with a pipette, spread evenly on a slide, and allowed to dry on a hotplate. A

coverglass was affixed to the slide with Caedax mounting medium, and the

slide was then reheated on the hotplate for 1 minute to harden the medium,

making the mount permanent. This method provided a fairly even

distribution of the constituents.

Visual estimates of the percent frequency of the fine-fraction

components (Table 2-2) were made by scanning the slide at 600x. Calcareous

nannofossils were observed to be the main constituent of the fine fraction. A

considerable amount of unidentified calcareous material were present and

associated with the nannofossils in every sample. Microforaminifers were

observed in most samples. Diatoms and radiolarians and related fragments

19

were relatively common in some samples. Overall, very little organic carbon

or unidentified minerals were present in the fine fraction.

The fine-fraction samples were checked for evidence of dissolution

and precipitation of calcite. Dissolution appeared to be insignificant in the

fine fraction. Preservation of the nannofossils was indicated as moderate

(Table 2-2) if some of the discoasters were coated with secondary calcitic

growth. Overgrowth did not appear to affect the coccoliths.

Samples of the fine fraction were also checked for age based on the

calcareous nannofossils. From this examination, we concluded that

reworked sediment was not a factor in the samples studied.

Results

Physical Properties

Downhole profiles of porosity, compressional wave velocity and

calcium carbonate content for the upper 300 m of Hole 803D are shown in

Figure 2-2. The profiles of porosity and velocity show general trends of

decreasing porosity and slightly increasing velocity with increasing depth.

These general trends are caused by increasing compaction of the sediment

column with increasing overburden.

Measured porosities for Hole 803D decrease at a fairly constant rate

with depth from near 75% at the seafloor to near 53% at 300 meters below

seafloor (mbsf). Bassinot et al. (1993a) concluded that mechanical compaction

is the major compaction process acting throughout the entire ooze-chalk

section down to about 600 mbsf, based on the lack of marked change in the

20

trend of the downhole porosity profile. The high-porosity spike in the 803D

profile at 181.20 mbsf corresponds to an approximately 0.5 em thick ash layer.

Measured downhole velocities in Hole 803D remain fairly constant

near 1550 meters per second (m/s) to approximately 140 mbsf. Between

approximately 140 mbsf and the bottom of the studied section at 300 mbsf,

measured velocities show more variability, fluctuating between 1,500 m/s

and 1,650 m/s. Shipboard scientists (Shipboard Scientific Party, 1991a) noted

that cementation began to be observed in the sediment column around 150

mbsf. The change in the velocity profile below this approximate depth likely

indicates the role of cementation in increasing compressional wave velocity,

due to increasing rigidity of the sediment fabric.

Measured calcium carbonate contents in these sediments are in the

range of 85 to 95%. CaC03 contents increase from about 85% near the

seafloor to around 90% at 50 mbsf, while CaC03 contents between 50 and 300

mbsf remain fairly constant with depth, fluctuating between 90% and 95%.

There are two low-carbonate excursions observed in the downhole profile.

The two low-carbonate spikes at 146 mbsf and at 181.20 mbsf correspond to

two ash layers, one of which was previously discussed for the porosity data.

Independent of the downhole trends, a number of small-scale

fluctuations with depth frequencies on the order of meters or tens of meters

are imprinted on the porosity, velocity and carbonate profiles (Figure 2-2).

The relationship of these small-scale fluctuations to differences in

microfossil content was the focus of this study.

21

Microfossil Content and Preservation

Grain size vs. depth for the samples analyzed is shown in Figure

2-3A. The fine fraction «63 urn) makes up more than 90% by weight of most

of the samples. The samples from the upper 50 m, however, contain 10% to

30% coarse particles (>63 urn).

The components of counted coarse particles are shown in Figure 2-3B.

Four components total more than 98% of the total coarse particles; the other

components of the coarse fraction are not plotted. "Planktonics" refers to

planktonic foraminifers with tests estimated to be ~50% intact. "Foraminifer

fragments" refers to both planktonic and benthic foraminifers with tests

estimated to be <50% intact. The planktonic fragments were estimated to

comprise 98% of the total fragments (i.e., the occurrence of benthic

foraminifer fragments was estimated to be 2%). Both an increase in the

frequency of radiolarians and/or an increase in the frequency of foraminifer

fragments and entire benthic foraminifers are recognized as a sign of

dissolution of carbonate in the sand fraction.

Numerous fluctuations were present in the relative percentages of the

major constituents. However, some general trends were observed.

Radiolarian content increases downcore; the samples near the surface

usually contain less than 5% radiolarians. The radiolarian content of the

majority of samples below 150 mbsf is greater than 50%. The percentage of

foraminifer fragments generally decreases downhole, from values greater

than 80% near the surface to less than 40% below 120 mbsf. The general

downhole trend of increasing radiolarian to foraminifer fragment ratio

presumably represents a gradual lowering of the lysocline over the time

22

interval represented by these sediments. The present-day deeper lysocline

results in less dissolution and a lower radiolarian to foraminifer fragment

ratio.

Planktonic foraminifers exhibit no general depth trend; this implies

that dissolution is sufficient to affect the relative number of fragments (and

the associated fragments to radiolarian ratio), but not to result in a significant

decrease in the frequency of whole foraminifers. Fragments appear to be the

particles most susceptible to complete dissolution following burial.

Imprinted on the general downhole trends observed in the coarse

fraction profiles are fluctuations of higher planktonic foraminifer content

and lower radiolarian counts (e.g., 138-145 mbsf and 190-198 mbsf). These

small-scale fluctuations in microfossil content presumably represent

variations in CaC03 dissolution caused by short-term changes in the depth of

the lysocline.

Nannofossils are the primary constituents of the fine fraction, usually

numbering greater than 80% of the fine particles (Figure 2-3C). Foraminifer

fragments generally number less than 5% of the fine fraction; unidentified

calcareous particles number between 5% and 10%; diatoms, radiolarians, and

unidentified siliceous particles generally number less than 3% of the fine

particles.

No obvious trends with depth were observed for the fine fraction

constituents. However, one notable interval occurs between 205 and 220

.mbsf, in which the number of various siliceous particles increases to a total

of approximately 20% of the fine particles. This interval corresponds to one

of higher radiolarian content in the coarse fraction and presumably

23

represents increased dissolution of CaC03 prior to burial (Le., a shallower

lysocline).

Correction of Downhole Profiles

Two types of signal are seen in the downhole physical properties and

microfossil data, namely (1) downhole depth trends due to compaction of the

sediment column (physical properties) or due to long-term changes in the

depth of the lysocline (microfossil content); and (2) small-scale fluctuations

presumably caused by short-term variations in dissolution. The intent of

this study was to examine the second signal, i.e., the relationships between

small-scale fluctuations in physical properties and microfossil content.

Therefore, we have corrected downhole data to remove the effects of the first

signal, i.e., compaction and long-term dissolution trends.

Second-order polynomial curves were fit to each physical property

dataset, as shown for the examples of downhole porosity and velocity data in

Figure 2-4. The curve-fit value of either density or velocity at the shallowest

sample depth was assumed to represent the "original" (uncompacted) value

(represented by the solid vertical line) and the shipboard data are corrected by

the difference between this uncompacted value and the curve fit value for

each sample depth. For example, the adjusted porosity (66.8 %) at 270 mbsf

equals the measured porosity (53.8%) plus the difference between the fit

porosity (55.8%) and the "uncompacted" value (69.5%).

We used the described procedure to subtract the effects of compaction

from the depth trends in all of the physical properties parameters utilized in

this study, i.e., porosity, velocity, and acoustic impedance. We have assigned

24

arbitrary values to the corrected data, so that they are not confused with the

measured physical properties data. For example, corrected porosities range

from a% to (a+25)%. This procedure to remove compaction effects assumes

that the compaction of the sediment column in the upper 300 m can be

approximated by the polynomial curve applied. This assumption is probably

an oversimplification, as some zones exhibit different consolidation

behavior dependent on their constituents (Chapter 3). However, the

polynomial approximation is suitable for our purposes.

Foraminifer fragments and radiolarians also show marked trends with

depth that likely result from post-depositional dissolution (Figure 2-5). We

have corrected the data for these two constituents employing the methods

used to correct for depth trends due to compaction in the physical properties

data. Corrected foraminifer fragment and radiolarian data are plotted vs.

depth in Figure 2-5. For each parameter, we have applied a polynomial fit to

the actual data and then shifted the data by the difference between the fit and

the "pre-dissolution" microfossil content (represented by the solid vertical

line). As was performed for the physical properties data, we have assigned

arbitrary values to the corrected microfossil data, so that they are not

confused with the measured data.

Planktonic foraminifer data were also corrected, although the slight

change in the measured data with depth makes the correction much less

significant than for the other two microfossil parameters. Application of

these corrections to all three microfossil parameters assumes that observed

downhole dissolution trends can be approximated by a polynomial fit.

25

Discussion

Role ofIntraparticle Porosity

Previous studies (e.g., Hamilton et al., 1982; Bachman, 1984) have

found that relationships between porosity and other properties, including

velocity, grain size, and CaC03 content, are difficult to establish for calcareous

sediments with high CaC03 contents. We have also found this to be true in

our study of Ontong Java Plateau sediments. Plots of porosity vs. weight

percent of coarse particles and versus abundance of individual microfossil

constituents (Figure 2-6) show no obvious relationships between porosity

and these parameters.

The difficulty in establishing relationships between porosity (or

density) and velocity, carbonate content, or microfossil content is largely

because of voids in the microfossil tests. Many authors (e.g., Schreiber, 1968;

Morton, 1975; Johnson et al., 1977) have found that relationships between

porosity and other physical properties are complicated by the fact that the

microfossil tests are hollow and produce higher porosity sediment than

similar-sized solid particles. Several studies (e.g., Gallagher, 1967; Johnson et

al., 1977; Hamilton et al., 1982) have indicated that calcareous sediments react

to the passage of sound waves as if they were composed of solid, rather than

hollow, particles. However, the methods used in the laboratory to determine

index properties measure the total sample void space.

This phenomenon results in a difficulty in determining relationships

between the index properties and other parameters in this study; it has also

plagued the studies of others. For example, Hamilton et al. (1982) found that

porosity and density are good indexes to velocity in the deep, eastern

26

equatorial Pacific, where calcium carbonate contents are lower, but they

found no usable relationship between velocity and density or velocity and

porosity in Ontong Java Plateau sediments. Hamilton et al. (1982) also found

little relationship between percent CaC03 and sound velocity or percent

CaC03 and bulk density for Ontong Java Plateau sediments. They found that

impedance has a linear relationship with saturated bulk density in the

eastern Pacific samples, but that the plateau samples diverge markedly from

this trend.

The total porosity is made up of intraparticle porosity, defined by

Choquette and Pray (1970) as porosity within individual grains or particles,

and interparticle porosity, defined as the porosity between particles (i.e.,

matrix porosity). In terrigenous sediments, consisting predominantly of

solid particles, porosity increases with decreasing grain size (Schreiber, 1968;

Johnson et al., 1977). As the grain size of foraminifer-rich sediments

decreases, and the number of broken or severely corroded tests increases,

intraparticle porosity is converted to interparticle porosity. However, total

porosity (as measured by shipboard procedures) remains fairly constant.

An estimate of interparticle porosity is needed for correlation with

acoustic and other properties (Bachman, 1984). Correction of total to

interparticle porosity is difficult because few estimates of that portion of the

porosity contributed by hollow foraminifers are available. Bachman (1984)

performed a study measuring the porosities of glass beads and foraminifers

of the same grain diameters (Figure 2-7). He classified the difference in the

two porosities for the same grain size as intraparticle porosity. Bachman's

27

work indicated that intratest voids can increase total sediment porosity by

44% at test diameters of 0 phi, and by 35% at diameters of 4 phi.

Urmos (1991) estimated intraparticle porosity for the Ontong Java

Plateau sediments of our study. These estimates of intraparticle porosity

were derived from analyses of porosity-velocity cross plots of downhole

logging and shipboard data. Urmos assumed that different porosities in

samples of the same velocity were the result of different amounts of

intraparticle porosity. We have used these estimates of intraparticle porosity

to calculate interparticle porosity for our samples.

Plots of interparticle porosity vs. percent coarse fraction, planktonic

foraminifers, foraminifer fragments, and radiolarians are shown in the four

plots of Figure 2-8. A comparison of these porosity vs. microfossil plots with

those of Figure 2-6, plotted as the same scale, show that the interparticle

porosity data exhibit considerably less scatter. The calculated interparticle

porosity vs. percentage coarse fraction shows a clearer relationship with grain

size than is observed with total porosity (Figure 2-6). Although the range of

porosities is small, a relationship of decreasing interparticle porosity with

increasing grain size is observed, similar to the relationships observed for

porosity and grain size in other sediment types (Johnson et al., 1977).

A relationship of decreasing interparticle porosity with increasing

number of whole planktonic foraminifers is also observed (Figure 2-6).

These relationships of interparticle porosity and grain size are anticipated, as

the volume of matrix (i.e., interparticle porosity) should decrease as the

relative percentage of large grains, namely whole planktonic foraminfers for

28

these sediments, increases. A relationship of increasing interparticle porosity

with increasing radiolarian content is seen in the plot of these parameters.

These relationships between interparticle porosity and microfossil

content may have been better-defined using another method of estimating

interparticle porosity, such as analyses of sample SEM images, or a statistical

estimate based on the observed number of intact microfossil tests.

Influence of Cementation

Plots of compressional-wave velocity vs, weight percent of coarse

particles and versus abundance of individual microfossil constituents

(Figure 2-9) show considerable scatter. The downhole velocity data showed

significantly more fluctuation below 150 mbsf (Figure 2-2). Shipboard

lithologic descriptions (Shipboard Scientific Party, 1991a) note a transition

from soft to stiff ooze at approximately 150 mbsf, coincident with incipient

carbonate cementation. A seismic reflection horizon is also located at this

depth (Shipboard Scientific Party, 1991a), which corresponds to the

approximate depth of a significant change in the relative numbers of the

coarse fraction microfossil constituents (Figure 2-3B). Since even a small

amount of cementation significantly increases rigidity, and thereby velocity,

velocity in the deeper sections is likely more affected by cementation than by

sediment constituents. Velocity data in stiffer oozes may also be affected by

increased measurement error due to cracking of the sediment upon insertion

of the DSV transducer probes (Bassinot et al., 1993b).

Plots of velocity vs. coarse fraction microfossil constituents for the

upper 150 mbsf (Figure 2-10) show a somewhat clearer relationship between

29

velocity and the microfossil parameters. Therefore, the significant scatter

observed in the velocity plots of Figure 2-9 is somewhat a result of scatter in

the data from depths below 150 mbsf.

Velocity vs. grain size (Figure 2-10) for samples in the upper 150 m

exhibits a relationship of increasing velocity with increasing grain size, as

observed in other studies (Schreiber, 1968; Mayer, 1979; Hamilton et al., 1982).

Johnson et al. (1977) also found that lower sound velocities and shear

strength corresponded to lower mean grain size of Ontong Java Plateau

surface samples. Our data show that increasing velocity corresponds to

increasing planktonic foraminifer content and decreasing foraminifer

fragment and radiolarian content (Figure 2-10). Therefore, our data indicate

that increased dissolution corresponds to a decrease in velocity. This

observation is in keeping with Johnson et al. (1977) who found that velocity

decreases with increasing water depth (i.e., increased proximity to the CCD

and, therefore, increased dissolution).

Finally, by using these restrictions on the calculated impedance, i.e.,

bulk density calculated from interparticle porosity and velocity data for the

upper 150 mbsf to exclude the effects of cementation on velocity, we can plot

impedance versus microfossil parameters (Figure 2-11). We observe a

relationship of decreased impedance with decreased grain size; this

relationship was also observed by Berger and Mayer (1977) for calcareous

sediments. Relationships of slightly increasing impedance with increasing

planktonic foraminifers and slightly decreasing impedance with increasing

radiolarian content (Figure 2-11) reflect the relationships observed between

these microfossil parameters and porosity and velocity.

30

Relative Significance of Fine Fraction Constituents

An interesting result of this study is that no relationships were found

between physical properties parameters and variations in the individual fine

fraction «63 urn) constituents. Plots of porosity, interparticle porosity,

velocity, and impedance (Figure 2-12) versus nannofossil content show no

apparent relationships. It appears that any variations in the physical

properties as a result of changes in microfossil constituents are controlled by

changes in the relative percentages of the coarse fraction constituents.

As indicated by the data shown in Figure 2-3B, the relative percentages

of the fine fraction constituents fluctuate very little compared to the relative

percentages of the coarse fraction constituents. Therefore, while the fine

fraction constituents make up greater than 80% of the total sample by weight,

fluctuations of the fine fraction constituents are minimal in these high­

carbonate sediments. The fluctuations that are observed also correspond to

fluctuations at the same depth interval in the coarse fraction data, and it

appears that the coarse fraction fluctuations control the physical properties.

Relationships with Other Shipboard Data

We also examined other physical shipboard data (such as grain size

and smear slide data) and post-cruise data (such as SEM photographs) to

determine if these data could be used in our study. We found that sediment

even a few centimeters away from our study samples could exhibit extremely

different physical properties and microfossil constituents than those

measured for our samples. It is apparent that the variations in downhole

physical properties and microfossil content occur at such a high frequency

31

with depth that relationships can only be established by looking at these

parameters for the same samples.

Effect of Post-Burial Dissolution

An issue to be addressed in relating seismic reflectors to preservation

as a result of shifts in the lysocline is the degree of dissolution that occurs

after burial. In a sediment system so highly saturated in CaC03, it is possible

that only minor, if any, dissolution of buried tests would occur until

overburden pressures became high enough for pressure solution at grain

contacts (may occur at the ooze-chalk transition), In addition, post-burial

dissolution and reprecipitation of calcite cement appears to be somewhat

related to the diagenetic potential of the sediment at burial (Schlanger and

Douglas, 1974). However, Berger and Johnson (1976) concluded that the

depth dependence of the number and spacing of reflectors suggests that

dissolution in the water column and at the sediment-water interface is the

primary agent controlling the profiles.

Preservation in this study was judged as moderate if some of the

discoasters were coated with secondary calcite growth. Overgrowth did not

appear to affect the coccoliths. Preservation is described as good in the upper

125 m; below that depth, it ranges from moderate to good. It appears that the

dissolution responsible for the fluctuations in microfossil content in the

sediment column occurred predominantly in the water column and at the

sediment-water interface.

32

Seismic Reflectors

Berger and Mayer (1977) concluded that the origin of seismic reflectors

must be tied to lithologic changes that cause quasicyciic impedance contrasts.

While our study has provided relationships between physical properties and

microfossil constituents, we are also interested in linking seismic reflectors

observed in these sediments (Shipboard Scientific Party, 1991a; Mosher et al.,

1993) to lysoclinal cycles. The identified seismic reflectors at site 803 are

superimposed on downhole profiles of impedance, porosity, calcium

carbonate, percentage coarse fraction, and percentage of three individual

coarse fraction constituents (planktonic foraminifers, foraminifer fragments,

and radiolarians) in Figure 2-13. Shipboard scientists (Shipboard Scientific

Party, 1991a) used Site 803 preliminary shipboard physical properties (i.e.,

density, velocity, and acoustic impedance), lithologic (i.e., grain size and

carbonate), and biostratigraphic (i.e., microfossil content) data to speculate on

the cause of the reflectors observed at Site 803. We have expanded that

analyses by including our microfossil content data.

In the central and eastern equatorial Pacific, physical properties

changes are directly linked to changes in carbonate content (Mayer, 1979;

Mayer et al., 1993). However, for most sediments of the Ontong Java Plateau,

variations in carbonate content are not large enough to cause changes in

porosity or velocity. Instead, porosity and velocity fluctuations appear to be

related to changes in grain size, which in tum are related to dissolution or

winnowing events (Mayer et al., 1993).

Shipboard scientists (Shipboard Scientific Party, 1991a) determined that

reflector 3-1, the youngest reflector identified, was associated with a small but

33

rapid increase in bulk density that corresponds to a carbonate content

maxima and a large increase in mean grain size. The high carbonate content

and large grain size associated with reflector 3-1 were suggested (Shipboard

Scientific Party, 1991a) to indicate increased preservation that results from

the greater abundance of whole foraminifer tests. Reflector 3-1, at 2.6 Ma,

was suggested to be related to fluctuations in carbonate preservation in the

western equatorial Pacific and possibly linked to the initiation of Northern

Hemisphere glaciation (Shipboard Scientific Party, 1991a).

The microfossil data (Figure 2-13) indicate that the reflector lies just

below a zone of increased microfossil preservation, indicated by a high at

25.52 mbsf in percentage sand and planktonic foraminifers, and a low in

foraminifer fragments. However, the microfossil data suggest that the

reflector is actually within a zone of transition from preservation to

dissolution, as the planktonic foraminifer content decreases quickly over the

interval from 25.5 to 33.1 mbsf.

Shipboard scientists (Shipboard Scientific Party, 1991a) suggested that

reflector 3-2, at 3.3-3.4 Ma, was also associated with a carbonate content

maxima, therefore indicating increased preservation of carbonate. No

shipboard grain size measurements were made over the interval of reflector

3-2. This reflector was suggested to be related to fluctuations in carbonate

preservation in the western equatorial Pacific and possibly linked to

increased activity of North Atlantic Deep Water (NAD'\V) (Mayer et al., 1986).

However, our data indicate that this reflector characterizes a dissolution

event, corresponding to local minima in carbonate content, planktonic

34

foraminifers, and foraminifer fragments, and local maxima in radiolarians

(in both the coarse and fine fractions).

Shipboard scientists proposed that the grain size decrease and

carbonate minima associated with Reflectors 3-a and 3-b (at 4.3 and 5.0 Ma,

respectively) represent periods of enhanced dissolution, associated with

peaks in carbonate preservation in the Atlantic, with the 5.0-Ma event

possibly related to the isolation of the Mediterranean (the Messinian

interval). Our microfossil data do not provide conclusive evidence

regarding the nature of these reflectors, as the nearby microfossil samples

were all at least 3 meters from the reflector depth.

Shipboard scientists proposed that Reflector 3-3, at 153 mbsf (8-8.1 Ma) ,

is characterized by low carbonate content and a grain size maximum, and

that this relationship between carbonate content and grain size indicate the

removal of nannofossils by current winnowing. The winnowing indicated

as the cause of the Reflector 3-3 was suggested to be a regional phenomenon

as it did not appear to correspond to a time of major paleoceanographic

change (Shipboard Scientific Party, 1991a). However, our data indicate that

Reflector 3-3 corresponds to a sharp decrease in grain size, with planktonic

foraminifers decreasing rapidly from 55.8% of the coarse fraction at 145.17

mbsf to 1.7% at 154.69 mbsf. A corresponding sharp increase in the relative

percentage of radiolarians in the coarse fraction indicates that this is a

dissolution event. We also observe a decrease in velocity at this depth; this

general relationship of decreasing velocity with decreasing grain size is

reported in the literature and is supported by our data (Figure 2-10).

35

Reflector 3-c, at 183 mbsf (9.72 Ma), was believed by shipboard scientist

to represent a dissolution signal characterized by low carbonate content,

small mean grain size, and low velocity. This reflector lies immediately

below an ash layer that results in marked lows in impedance and carbonate

profiles (Figure 2-13). Reflectors 3-c, apparently a dissolution event, was

suggested to be coincident with a reflector extant throughout the central

Pacific that has been linked to global oceanographic events and, in particular,

major changes in NADW circulation (Mayer et al., 1986).

Reflector 3-4, at approximately 215 mbsf (12.3-12.5 Ma), was determined

to suggested by shipboard scientists to a velocity, density, and carbonate

minimum, whereas grain size data appeared to be ambiguous (i.e.,

undergoing a small change from fairly coarse- to fine-grained sediments).

Similarly to Reflector 3-c , Reflector 3-4 was suggested to be a central Pacific

dissolution event linked to global oceanographic events and, in particular,

major changes in NADW circulation (Mayer et al., 1986).

Our microfossil data do not provide conclusive evidence regarding the

nature of reflector 3-4; the microfossil data indicate a transition zone with

increasing foraminifer fragment to radiolarian ratio. The ooze-chalk

transition was placed by shipboard sedimentologists several meters below

Reflector 3-4. This transition is clearly seen in the sharp change in slope of

both the velocity and density curves (Figure 2-2), resulting from increasing

cementation. It is possible that variations in cementation, rather than grain

size and microfossil constituents, control the impedance contrasts that would

result in reflectors at this depth.

36

Reflector 3-5, at approximately 240 mbsf (15.8 Ma), was suggested to be

related to the rapid changes in velocity in this part of the section, resulting in

high-frequency changes in the degree of cementation. However, shipboard

scientists found it difficult to speculate on the origin of these velocity

changes, as the carbonate and grain size data in this depth interval were

limited. Our microfossil data indicate that the reflector also corresponds to

sharp decreases in the relative percentages of planktonic foraminifers and

foraminifer fragments, and a sharp increase in radiolarian content.

Reflector 3-5 lies at the end of a major hiatus (Shipboard Scientific

Party, 1991a) that was suggested to correspond to a widespread hiatus Keller

and Barron (1983) linked to major changes in ocean circulation and seismic

events (Mayer et al., 1986). The large contrast in age across this stratigraphic

break was suggested to result in a rather sharp jump in the state of

induration of the sediment and thus the velocity and density structure. Our

microfossil data indicate that this reflector is also characterized by a

dissolution event.

Deeper reflectors and reflector packages were also discussed by the

shipboard scientists, although correlations to grain size and carbonate data

appeared to be less definable. Deeper reflectors may be related to changes in

cementation, rather than to fluctuations in grain size or microfossil content.

Conclusions

Relationships between sediment microfossil content and physical

properties are observed. However, the role of intraparticle porosity in these

highly calcareous sediments makes the use of index properties data as

37

habitually determined on the ship inappropriate. Calculation of interparticle

porosity provides relationships between interparticle porosity and

microfossil constituents or grain size. Strong relationships are not observed

in this study because of the small range of values for most of the physical

properties parameters, including porosity, for this site. In addition, the

scatter observed in these data could likely be reduced with better estimates of

interparticle porosity.

Velocity data below 150 mbsf show significant scatter,likely due to the

influence of incipient cementation near this depth. Clearer relationships are

observed for velocity data for depths shallower than 150 mbsf. Using the

corrections for interparticle porosity, impedance data for samples in the

upper 150 mbsf were found to increase with increasing grain size and

planktonic foraminifera content.

The fine fraction « 63 urn) constituents make up greater than 85% of

the samples by weight. However, no relationships are observed between

variations in fine-fraction constituents and variations in physical properties.

It appears that the variations in the coarse-fraction constituents have a more

significant effect on the physical properties.

Many of the seismic reflectors identified by shipboard scientists could

be related to changes in the relative percentages of microfossil constituents.

For sediment below the ooze-chalk transition, fluctuations in cementation,

rather than fluctuations in grain size or microfossil constituents, may result

in impedance contrasts and seismic reflectors.

38

Table 2-1. Coarse fraction constituents for Hole 8030; coarse (>631lm) particles expressed as weight percentage of total sample, andindividual constituents of coarse fraction expressed as percentage of number of total coarse particles.

"Coarse fraction Planktonic Benthic Foraminifer

Depth (% total foraminifers foraminifers fragments Radiolarians Diatoms Other Minerals(rnbsf) sample weight) (% coarse) (% coarse) (% coarse) (% coarse) (% coarse) (% coarse) (% coarse)

5.07 11.3 9.5 1.7 85.1 3.3 a 0 0.4

6.49 13.4 15.1 1.6 79.8 3.5 0 0 0

9.46 15.8 3.2 2.2 91.4 2.8 0 0.2 0.220.59 20.9 17.8 0.9 79.6 1.5 0 0.1 0

24.10 21.6 32.5 0.7 65.8 1.0 0 0 0.1

25.51 30.4 38.7 0.2 59.3 1.9 0 0 030.79 22.9 15.8 0.5 81.6 2.0 0 0.1 033.11 20.9 2.0 0.9 88.7 8.0 0 0.3 0

IJJ 38.10 16.4 16.8 1.1 81.6 0.3 a 0.2 0\0

39.60 9.6 7.1 1.2 78.3 13.2 a 0.1 0.143.36 10.8 17.5 1.1 77.7 3.6 0 a 0.144.89 15.3 11.5 0.7 80.0 7.6 0 0.1 046.35 7.8 8.8 1.8 82.6 6.7 0 0 0.1

46.46 10.6 8.8 1.3 81.9 7.6 0 0.3 a48.70 6.9 4.2 1.0 88.4 6.2 0 0.3 051.70 8.8 55 1.0 76.4 16.9 0 0 0.362.70 8.4 8.5 0.6 68.7 22.0 a 0.2 073.68 5.2 6.5 1.1 70.8 21.1 0.3 0 0.275.60 5.1 19.1 1.1 57.3 22.3 0.1 0 0.177.17 3.9 11.0 1.4 69.3 18.0 0.1 0.3 083.19 7.6 13.8 0.5 73.7 11.7 0.1 0.1 0.1

83.95 5.3 7.8 1.1 70.7 20.0 0 0.1 0.2

Table 2-1 (continued). Coarse fraction constituents for Hole 8030; coarse (>63 urn) particles expressed as weight percentage of totalsample, and individual constituents of coarse fraction expressed as percentage of number of total coarse particles.

Coarse fraction Planktonic Benthic ForaminiferDepth (% total foraminifers foraminifers fragments Radiolarians Diatoms Other Minerals(mbsf) sample weight) (% coarse) (% coarse) (% coarse) (% coarse) (% coarse) (% coarse) (% coarse)

88.18 8.7 25.0 0.7 53.1 21.0 0.1 0 0.1

89.69 6.8 11.1 0.9 61.6 25.5 0.9 0 0.1

93.46 5.4 6.9 1.2 65.1 25.7 0.5 0.2 0.4

94.70 4.2 4.9 0.6 58.0 35.8 0.4 0 0.4

100.70 3.8 12.9 1.3 58.0 27.5 0.1 0 0.1

107.17 2.4 18.8 0.6 41.8 38.6 0 0 0.2111.68 6.1 13.7 1.0 73.9 11.4 0 0 0115.14 4.8 12.2 0.4 45.5 41.2 0.1 0 0.6

""- 118.51 2.9 17.6 0.8 39.7 41.7 0 0.1 0.1a

15.2119.58 3.2 0.6 38.6 45.1 0.1 0 0.4123.20 3.2 25.9 1.5 39.4 32.6 0 0 0.6

124.68 4.4 29.7 0.3 41.8 28.2 0 0 0127.69 3.4 16.9 1.2 46.0 35.5 0 0 0.4

129.13 6.6 38.5 0.4 32.5 27.4 0.8 0 0.4135.70 4.7 12.7 0.8 47.3 38.4 0.8 0 0138.68 12.9 68.1 0.1 16.9 14.8 0 0.1 0140.19 14.3 60.9 0.3 22.7 16.1 0 0 0

145.17 14.1 55.8 0.1 19.7 24.3 0.1 0 0148.20 8.3 25.5 0.3 37.5 36.1 0.3 0.2 0149.59 5.6 13.2 0.6 28.7 57.5 0 0 0154.69 3.6 1.7 1.6 23.8 72.8 0 0.1 0.1

162.70 5.5 16.1 0.6 45.4 37.7 0 0.3 0

Table 2-1 (continued). Coarse fraction constituents for Hole 8030; coarse (>63 11m) particles expressed as weight percentage of totalsample, and individual constituents of coarse fraction expressed as percentage of number of total coarse particles.

Coarse fraction Planktonic Benthic ForaminiferDepth (% total foraminifers foraminifers fragments Radiolarians Diatoms Other Minerals(mbsf) sample weight) (% coarse) (% coarse) (% coarse) (% coarse) (% coarse) (% coarse) (% coarse)

167.20 2.4 5.4 1.5 35.3 57.8 0 0.1 0

172.20 1.8 2.4 1.5 24.3 71.1 0 0.2 0.6

176.69 1.5 6.7 1.3 34.3 57.6 0 0 0

178.89 1.5 3.1 1.0 36.0 59.2 0 0.4 0.2

184.68 4.5 3.1 2.3 60.4 33.8 0 0.2 0.2

186.20 3.8 3.7 0.7 38.6 56.9 0 0.1 0189.64 10.4 46.4 0.3 43.7 9.4 0 0 0.1

197.99 6.7 40.7 1.0 34.4 23.7 0 0 0.3

~199.09 4.6 18.8 1.0 32.7 47.5 0 0 0.....205.09 4.5 12.3 1.1 22.2 64.0 0 0.2 0.2

208.73 9.5 7.3 0.2 32.2 60.2 0 0 0.1216.13 13.0 9.0 0.2 47.7 42.9 0.1 0 0.1218.15 6.7 6.3 0.1 38.5 54.7 0.1 0 0.3

220.81 10.3 8.7 0.6 56.8 33.6 0 0 0.3228.56 8.8 40.4 0.5 42.4 16.6 0 0 0

231.92 2.9 2.8 0.4 13.5 83.2 0 0 0237.23 11.0 36.1 0.2 34.7 28.9 0 0 0.1

256.14 0.6 5.3 2.0 14.9 77.4 0 0.2 0.2259.09 2.6 46.2 0.3 33.1 20.1 0 0.3 0

261.79 5.2 23.9 0.8 37.6 37.6 0 0.2 0267.31 3.2 49.5 0.5 34.0 15.5 0 0 0.5270.30 0.9 2.0 0.8 3.0 93.9 0 0.3 0

Table 2-2. Fine fraction constituents for Hole 8030; individual constituents of fine fraction expressed as percentage of total fineparticles.

Unidentified UnidentifiedNanno- Foraminifer calcareous Radio- Organic Unidentified silicious

Depth fossils fragments particles larians Diatoms carbon minerals particles Preservation(mbsf) (%) (%) (%) (%) (%) (%) (%) (%)

5.07 89 3 7 <1 <1 <1 <1 <1 Good6.49 89 4 6 <1 <1 <1 <1 <1 Good

9.46 90 3 5 <1 1 <1 <1 <1 Good

20.59 88 2 8 <1 <1 <1 <1 1 Good24.10 88 3 8 <1 <1 <1 <1 <1 Good25.51 89 5 5 <1 <1 <1 <1 <1 Good

30.79 87 5 6 <1 <1 <1 <1 1 Good33.11 87 5 5 <1 1 <1 <1 1 Good

>1>038.10 88 6 5 <1 <1 <1 <1 <1 GoodN

39.60 88 5 5 <1 1 <1 <1 <1 Good43.36 65 <1 25 <1 <1 0 9 <1 Good44.89 88 3 6 <1 1 <1 <1 1 Good

46.35 83 3 10 <1 <1 <1 3 <1 Good46.46 89 4 4 <1 1 <1 <1 1 Good

48.70 83 5 10 <1 <1 0 1 <1 Good

51.70 80 1 15 <1 2 0 2 <1 Good

62.70 86 2 9 <1 1 <1 <1 1 Good73.68 85 <1 12 <1 <1 <1 <1 <1 Good75.60 85 1 10 1 2 0 1 <1 Good

77.17 86 1 10 <1 1 <1 <1 1 Good

83.19 85 1 10 1 2 0 1 <1 Good83.95 85 <1 8 2 2 0 <1 2 Good

Table 2-2 (continued). Fine fraction constituents for Hole 803D; individual constituents of fine fraction expressed as percentage of totalfine particles.

Unidentified UnidentifiedNanno- Foraminifer calcareous Radio- Organic Unidentified silicious

Depth fossils fragments particles larians Diatoms carbon minerals particles Preservation(mbsf) (%) (%) (%) (%) (%) (%) (%) (%)

88.18 87 1 8 1 1 0 <1 2 Good

89.69 87 1 7 2 1 0 <1 1 Good

93.46 86 1 8 2 2 0 <1 <1 Good94.70 88 1 5 1 2 <1 1 2 Good

100.70 87 1 7 1 1 0 <1 2 Good107.17 87 1 7 1 1 0 1 2 Good

111.68 88 2 7 0 2 <1 <1 <1 Good

115.14 88 4 4 0 3 0 <1 <1 Good"'"~ 118.51 87 1 7 2 2 <1 <1 <1 Good

119.58 87 4 6 0 2 0 <1 <1 Good123.20 89 2 4 0 4 0 <1 <1 Good124.68 82 5 9 <1 2 <1 1 <1 Good

127.69 80 1 16 <1 1 <1 1 <1 Moderate/Good

129.13 82 5 7 <1 5 <1 <1 <1 Good135.70 82 4 6 <1 4 <1 2 1 Moderateate

138.68 81 10 7 <1 1 <1 <1 <1 Good

140.19 84 4 9 <1 1 <1 <1 <1 Moderate

145.17 81 3 12 0 2 0 1 1 Moderate/Good

148.20 85 3 8 <1 3 <1 <1 <1 Moderate/Good149.59 80 2 14 <1 2 <1 1 1 Moderate/Good

154.69 84 3 8 <1 4 <1 <1 <1 Moderate/Good

162.70 86 3 7 <1 3 <1 <1 <1 Moderate

Table 2-2 (continued). Fine fraction constituents for Hole 8030; individual constituents of fine fraction expressed as percentage of totalfine particles.

Unidentified UnidentifiedNanno- Foraminifer calcareous Radio- Organic Unidentified silicious

Depth fossils fragments particles larians Diatoms carbon minerals particles Preservation(mbsf) (%) (%) (%) (%) (%) (%) (%) (%)

167.20 86 2 8 <1 3 <1 <1 <1 Moderate/Good

172.20 87 2 8 <1 1 0 0 1 Good

176.69 86 1 9 <1 2 0 <1 1 Good178.89 86 2 8 <1 3 <1 <1 <1 Moderate/Good

184.68 87 1 9 <1 2 0 <1 1 Good

186.20 88 1 8 <1 2 0 <1 1 Good

189.64 87 1 9 <1 1 0 <1 1 Moderate/Good

197.99 88 1 8 <1 <1 0 <1 2 Moderate/poor

t 199.09 88 1 5 1 3 0 <1 1 Moderate

205.09 70 1 10 2 10 0 <1 7 Moderate/Good

208.73 70 1 7 1 10 0 1 10 Moderate/Good

216.13 75 <1 5 2 10 0 1 7 Moderate/Good

218.15 75 1 4 3 10 0 <1 7 Moderate

220.81 80 1 3 2 8 <1 <1 5 Moderate

228.56 80 2 10 1 5 <1 <1 1 Moderate

231.92 82 2 8 1 5 <1 <1 1 Moderate/Good

237.23 82 3 8 1 5 <1 <1 <1 Moderate/Good

256.14 84 3 9 <1 2 <1 <1 1 Moderate/Good

259.09 85 5 9 <1 <1 0 <1 <1 Moderate/Good

261.79 85 4 6 <1 2 0 <1 2 Good267.31 84 5 7 <1 2 0 <1 1 Good270.30 85 4 8 <1 1 <1 <1 1 Good

Figure 2-1. Synthetic seismogram (center) flanked by field seismic record for

Site 803. Labels 3-1 through 3-11 and 3-a through 3-c correspond

to identified seismic reflectors.

45

2150 2155 2200 2205 2210 2215 2220

Field recordField recordO.9'-:::---'"'"'-'=~=:';;"::::==

eCI>

,§a;> 0.4~>.

'"~6~

46

Figure 2-2. Downhole profiles of porosity, compressional-wave velocity,

and calcium carbonate for the upper 300 m of Hole 803D.

47

50

csco, (%)

80 9070

Porosity (%)

55 65 75

! I ! ·f~

100 ~_.L...l.._..i>_]__L.__.

I i ~i I

i 1 { 1 i: : -: : :

i l: i ; ~

250 ~····~·:j~·····1"·······I········I········

l~l ! ! !: e: : : :i ~~ iii: .: : : :

IG~ I I I

150

48

Figure 2-3. (A) Grain size vs. depth for the samples analyzed in this study.

Coarse (sand-sized) fraction is the percent by weight retained on

a 63-~m sieve. (B) Relative percentages of the four major

components of the coarse fraction. (C) Relative percentages of

the fine fraction constituents.

49

Diatoms

Foraminiferfragments

Fine fraction(cumulative %)

20 40 60 80 100

Unidentifiedcalcareousparticles

Nannofossils

Radiolarians andunidentified

siliceous particles

o

cCoarse fraction(cumulative %)

20 40 60 80 100oB.

Sand-sizedfraction

Silt/clay-sizedfraction

50

Particle fractionA. (cumulative wt %)

o 20 40 60 80 100O,......,.--r--.--,...-,.-,......,...;r-,.~

100

..r:..go 150Q

­.....CI.l,.Q

E-

50

Figure 2-4. Correction of downhole porosity and velocity data to remove

the general depth trend caused by sediment compaction effects.

Shown are measured data (dashed line) and corrected data

(heavier solid line). Adjusted data (bottom scales) are reported

in increments of an arbitrary number plus % porosity or m/s

velocity, to differentiate corrected from measured data.

51

1700

. .T-

...........~ ~ .

Velocity (m/s)

1500 1600

... ~

------__.~t···:~.," :, :..

~ ~+100 ~+200

Velocity(carr) (mls)

••••••••••• "=" &O ••~ ••••••••

140085

a +20

Porosity (%)

65 75

a a+10Porosity(corr) (%)

50

250

200

100

150

52

Figure 2-5. Correction of downhole microfossil data, i.e., foraminifer

fragments and radiolarians, to remove the general depth trend

caused by dissolution effects. Shown are measured data (dashed

line) and corrected data (heavier solid line). Adjusted data

(bottom scales) are reported in increments of an arbitrary

number plus % microfossils, to differentiate corrected from

measured data.

53

50

100

--CI.l.0e-..c....

1500.. sll.lQ

200

250

I.. 1..+20 1..+40 1..+60Foraminifer Fragments(corr) (%)

54

Radiolarians(% of coarse fraction)

o 20 40 60 80 100

'I' +40 'I'+80 'I'+100Radiolarians(corr) (%)

Figure 2-6. Total porosity vs. percentage of coarse fraction and three coarse

fraction constituents (planktonic foraminifers, foraminifer

fragments, and radiolarians).

55

0.+15I • • • ..

•• • • e • -.. • •- •• 0 •~ • •• 0 • • • • • •0 0.+10- • • - • 0' • ••t \I. e\.~ e • '.'!'4 G ,0 • •• 0 • CI .. .. •u• e (i ...... , ·0£ fl • . .-\.-.. "- •....

til0. +5 •• • •0

I-c0

Pot

J0 •

1 L~ ,J 0 " 0 I 0 0 0 0 I 0 0 0 0 I 0 0 .: I 0 0 0 0 I 0 0 , 0 I 0 , 0 0 I, I , , I , , I , , , . . I I , I I ,

~ ~ +5 ~+1O ~+15 ~+20 {jJ m+20 m+40 {jJ +60 (jJ +80

Sand(corr) (%) Planktonic Foraminifers(cord (%;

010-

0.+15 ,• • • •

• •• e • • • •I • • • •- •?f? • • .. .41 • • .I • •- 0. +10 • • • •• .~-c -ec, -- •• . ~....... • •0 e· · '. . . . • ••• • •• •u

• 08 .0 .1. l·-·£ • • • • •.... e.- . . -til

0.+5 • • • •0I-c0

Pot

on [0 0 0 • I 0 ,~ , I , , , • I , ~, , I , , , , I , ••• 1 t"",,~ ,',' ,,' ,.""""""""" ,~" IA 1..+20 1.+40 1.+60 'I' '1'+20 'I'+40 'I' +60 'I'+80

Foraminifer Fragmenls(corr) (%) Radiolarians(corr) (%)

Figure 2-7. Total porosities of foraminiferal assemblages and glass beads.

Vertical lines indicate 95% confidence limits associated with the

plotted point. The internal porosity, or intratest porosity, is the

difference between the porosities of foraminifers and glass beads

for particles of the same diameter. Taken from Bachman (1984).

S7

-90 t- t t

*'l- FORAMINIFERA

-~

80-en

OJ 000 0::

45 f0 GLASS BEADSilNTERNAL POROSITY

a.. .. I - +

51 . 2 3 , 4

GRAIN DIAMETER, del> (PHI UNITS)

35 I , I I I I I

o

Figure 2-8. Interparticle porosity vs. percentage of coarse fraction and three

coarse fraction constituents (planktonic foraminifers,

foraminifer fragments, and radiolarians). Lines through the

data are least-squares fit, with equations shown.

59

ct+l01-

\"Porosity =67.0 - 0.07 (sand) IR=O.5

(jJ +80

•

(jJ +60

•

(jJ +40

Porosity =66.7 - 0.026(forams)R=O.5

(jJ +20

Planktonic Foraminifers(corr) (%)

•

• •• •~ . . .~ ". . ­.,. . ,

(jJ~+20

I

~+15

I

!;+10

Sand(corr) (%)

I

!; +5

Jj ••. ~.".~... . ..~ '.. . .. ,

• • lilt.I- •

Cl •

!;ct

ct +5

1:ou

~....Uleop...Q)

ut!res~J.<Q)....~-

-"#-

0\o

I

'1/+80

•

'1/+20 '1/+40 '1/+60

Radiolarians(corr) (%)

Porosity = 66.3 + 0.02(rads)• R=O.5

•

I .... I."." I ... " I .... I .... I .... I

I-

• • • •• .....~S;.,. .~. •1-. •• • • •• #

'1/

• ••

•

Porosity = 68.1 - 0.02 (fragments)R=0.2

•

A. +20 A. +40 A. +60

Foraminifer Fragments(corr) (%)

•

• Q ..... '~~"I· .,. , ... .• I) ••

A.ct

CL +5

CL +10

'Eou

~....Uleo~

Q)

u....1::res0..J.<Q)....~

>-<

_ ct+15 1"#- I "

Figure 2-9. Compressional-wave velocity vs. percentage of coarse fraction

and three coarse fraction constituents (planktonic foraminifers,

foraminifer fragments, and radiolarians).

61

P+200

~ • •~ P+150 • • • •6 ••• ,.

- Cl e •• • •• •'E ...- At •• •• •

e p+100 • • •• fI*! ... O••C \~ .e.......... .. , ,. ,: _, . . ...... e III •• ... ,. •• •u ••

~ P+50· •~

P L""".!.""", ,·1 f"""",!"""",!""!""!,,,,!,,,,!~ ~+s ~+10' ~+lS ~+20 (iJ m+20 m+40 m+60 m+80

Sand(corr) (%) Planktonic Foraminifers(corr) (%)0\N

P+2oo~ • •

- • 0~ P+lS0 • •

6 .... • •- .. .. .~. e. • •~ n 100 G.·. II" • • ~.. ••• •u ,.+ • fI' • ••~ · · • ¥4"~~'. e. •• e • • ,) .....'... ... ... ... -.u •• •

~ P+SO • •~

P f, ,,,,,,,, I , , , , , , , • , I , , , , , , , , ,I. f, ,,,,,,,,,,,,,I, , , , , , , , , I , , , , , , , , , I , , , , !

A A +20 A+40 A+60 'I' '1'+20 '1'+40 '1'+60 0/+80

Foraminifer Fragments(corr) (%) Radiolarians(corr> (%)

Figure 2-10. Velocity vs. percentage of coarse fraction and three coarse

fraction constituents (planktonic foraminifers, foraminifer

fragments, and radiolarians) for sample depths <150 mbsf. Lines

through the data are least-squares fit, with equations shown.

63

p+200<150mbsf <150mbsf

(jJ (jJ +40 (jJ +60 (jJ +80

Planktonic For.<tminifers(corr) (%)

(jJ1;+201;+15

e- • •• ,~ _O.}:;, , •

•

• •• •

•

1;+5 1;+10

Sand(corr) (%)

Velocity =1531 + 1.04 (sand)R=OA9

. 0-••

0..~D _

P I;

~ P+150El-'E.§. PtWO

.e-....uo

Q1 P+50:>

~

'l' +80

<150mbsf

I .... I .... I .... I .... I .... I

••• •• • •• ~•.,..-=------. ,)~---... -• • • •

Iv elOCity =1530 - 0.18 (rads)R=O.l

I-

I-

I-

'l' 'l' +20 'l' +40 'l' +60

Radiolarians(corr) (%)

<150mbsf

],,+20 ],,+40 ')..+60

Foraminifer Fragments(corr) (%)

ell ••e. •. ", •--...---• .J\.L4-~(. • ••,.. .... ..

"

I I , I I I , I I I I , I I , I , , I , I • , , , !Ilt""

P+50

p+100

--~ P+l50S'i:..ou

~oW....Uo­QJ

;>

P+200

Figure 2-11. Impedance vs. percentage of coarse fraction and three coarse

fraction constituents (planktonic foraminifers, foraminifer

fragments, and radiolarians) for sample depths <150 mbsf. Lines

through the data are least-squares fit, with equations shown.

65

o+0.6~ I ,IImpedance =2.4 + 0.004 (sand)R=0.5

• •

Impedance = 2.46 + 0.001 (forams)R=O.4

•• •• II

••

(jJ +20 (jJ +40 (jJ +60 (jJ +8U

Planktonic Foraminifers(corr) (%)

(jJ

•ii••

~ +5 ~+10 ~+15 ~+20

Sand(corr) (%)

•(l • •

•, .

~o

0+0.2

0+0.4

-;:..ou

OJ~

Uc:::l1S

resOJPo.8

1-4

­....'Ill

N

'8ueo

It)

ot'"4~

[Impedance = 2.54 - 0.001 (fragments)IR=0.25

Foraminifer Fragments(corr) (%)

'" +80

Impedance =2.48 - 0.001 (rads)R=0.3

• •.. .- I • •

• ! ••,- if. •• ••

'" +20 '" +40

Radiolarians(corr) (%)'"A+60

II

A +40II

A+20I

o. .•· l :t.f18

•• ) ...\' I.- • 1/1• 8 •

0\0\ - 0+0.6....,

IIIN,S

~

ueo

II') 0+0.41-0t'"4><- t--;:...0u

OJ~ 0+0.21-uc:::l1S

res I-OJPo.8 a

1-4 A

Figure 2-12. Total porosity, interparticle porosity, velocity and impedance vs.

relative percentage of nannofossils (expressed as a percentage of

fine fraction).

67

0.+15 _ 0.+15

I eft.-• 1: I •- • • • 0

~ • II ••u

~ 0.+10 • ~ 0.+101--;:; •• ....

,. ::11' til.. 00U J.o

a +5 f . .. I ·~ • 1.1.0 •.... III • It • 1.:0

•11 I-....

til • 0QJ

0 0. +5 - • •J.oU....

0 'tP-4 III •••• frQJ

0.' 1....

«I! , , , I ! , . , ,! , ! , ~ , , ! ! I ! ! ! ! !.....

60 70 80 90 60 70 80 9U

Nannofossils (% fine fraction) Nannofossils (% fine fraction)o-,oe

P+200 ;:;- ~+O,6

• I

• tilM•- • • • •• a

til p+150 • u •- bO •a •••••• 10 ~+0.4 •- • • • 0 •'E • '·1 ~ ••8 P+100 • : Ift.ll•• - •£ 1: ••."110.... u • ... I.. Iu • QJ"" HO,20 u .. ••• •- P+5U l:: •QJ • •> III

rt:J •QJ •Pro I

c, •, , , J ! I , I,

! I , , 1 Ei s70 80 90 1-4 60 70 8U 90

Nannofossils (% fine fraction) Nannofossils (% fine fraction)

Figure 2-13. Identified seismic reflectors at site 803 superimposed on

downhole profiles of impedance, calcium carbonate, and

percentage of three individual coarse fraction constituents

(planktonic foraminifers, foraminifer fragments, and

radiolarians).

69

Planktonic Foraminifers Radiolarians Foraminifer Fragments(% of coarse fraction) (% of coarse fraction) (% of coarse fraction)

v vv /v lOO 0 20 40 60 0 20 40 60 SO 100 0 20 40 60 80 lOOiii , iii'::t&J iii' iii iii iii iii iii Ii' i • Iii' iii

300 I , , ! I , ' C I , '

50

100

Qtil

..0S-o£i 150 I- ~ I t: --- I ~ I 1= ====---- I 1= r- 13-3$ .... .

"'J p..a Q,/

0

'oJ > t=J

I lC It~ I t )= 3-c

;i 7/'

3-4

3-5~ . <, I t r I ~ .7 I ~ <; I ~ 7 I

250

CHAPTER 3

INFLUENCE OF MICROFOSSIL CONTENT

ON CONSOLIDATION PROPERTIES

Introduction

Calcareous sediments have been reported as exhibiting unique

engineering and compression behavior that can be related to the type and

preservation of their major microfossil constituents (Demars, 1982). For

example, carbonate sediments do not compact to as Iowa void ratio as

noncarbonate sediments (Morelock & Bryant, 1971; Rezak, 1974). Several

studies (e.g., Morelock and Bryant, 1971; Kelly et al., 1974; Nacci et al., 1975;

and Valent et al., 1982) have focused on the physical behavior of marine

sediments and the relationship of various physical property parameters to

calcium carbonate (CaC03) content. Many of these investigations examined

sediments with CaC03 contents ranging from 30% to 90% and found distinct

correlations between specific parameters and CaC03 content.

Ontong Java Plateau sediments generally exhibit CaC03 contents near

90% (Figure 3-1). In a comparison of CaC03 content profiles for the five sites

drilled during ODP Leg 130, we observe that the sites at greater water depths

(Le., Sites 804 and 803) have carbonate contents generally near 90%, whereas

shallower sites (i.e., 806 and 807) have carbonate contents near 95% (Kroenke

et al., 1991). Increased calcite dissolution deeper in the water column results

in sediments of deeper sites exhibiting more CaC03 dissolution than

sediments of the same age at shallower sites. A schematic showing the

71

relative water depths of the five sites drilled during ODP Leg 130 is presented

in Figure 3-2.

This study examines the consolidation behavior of these high­

carbonate Ontong Java Plateau sediments, and attempts to relate the

observed behavior to sediment composition. In this study, we consider the

effects of carbonate content, grain size, intraparticle water, and cementation

on the observed consolidation behavior. Because the carbonate contents are

fairly uniform, we have also examined the data for indications that

variations in microfossil content (i.e., the relative abundance of intact

foraminifers) result in variations in the physical properties measured.

Consolidation Theory

Consolidation tests are used to assess the behavior of sediments under

mechanical loading. The process of consolidation involves the expulsion of

pore fluid and adjustment of the sediment structure as a result of the applied

stress. The rate and degree of consolidation have been observed (Demars,

1982; Valent et al., 1982) to vary for different sediment types and are

influenced by factors such as the depth and rate of burial, permeability of the

sediments, and the presence of diagenesis or cementation.

Consolidation involves two processes, the expulsion of pore fluid and

the adjustment of the sediment grains, that result in a decrease in sediment

volume with increasing overburden. One-dimensional consolidation

theory was first proposed by Terzaghi (1923). When a load is applied to the

saturated sediment, it is first carried by the pore fluid, resulting in an

instantaneous initial increase in pore pressure. Drainage is allowed to occur,

72

however, and dissipation of excess pore pressure occurs at a rate dependent

upon the permeability of the sediment, which is in turn affected by such

factors as sediment grain type, size and shape, number of connected pore

spaces, presence of cementation and diagenesis.

Primary consolidation is the consolidation stage characterized

predominantly by expulsion of pore fluid and subsequent reduction of pore

volume. As the excess pore pressure caused by a particular load dissipates,

the sediment grains assume the load, and adjust to a more compact structure.

This stage is referred to as secondary consolidation. The combined processes

of primary and secondary consolidation result in a reduction in sample

volume. The sediment grains are relatively incompressible, and the rate of

secondary consolidation depends mainly on grain type and cementation.

Primary consolidation (i.e., reduction of the pore volume) usually plays the

more significant role in total volume reduction, particularly in the short

term.

The one-dimensional consolidation test, in simple terms, consists of

the application of normal (vertical) pressures upon a small (often 6.2-cm

diameter), free-draining, confined, cylindrical sample. Loads are increased by

specified amounts and the rate and amount of volume decrease under each

load are recorded. Research has found that the best results are obtained

when the load is doubled producing a ratio of tlp/p =1 (Leonards, 1962).

Because the sample diameter is fixed, the change in sample volume can be

calculated from a measured change in sample height. Incremental loads are

applied and the change in sample height is measured with time. The results

of the tests are usually displayed as a plot of void ratio (e) versus the log of

73

effective vertical stress (log p'). The resulting plot is commonly referred to as

an e -log p' curve. Void ratio expresses a relationship between the volumes

in a soil occupied by solids and nonsolids. Void ratio (e) is calculated as

e = Vv / V s, (3-1)

where Vv is the volume of voids, Vs is the volume of solids, and e is

expressed as a decimal.

The semilogarithmic plot of e versus log p: for cohesive soils has the

following characteristics:

1) the initial branch of the curve, representing reloading of the

sample, has a relatively flat slope due primarily to the initial

loading being small pressure increments that are less than the

in situ effective overburden pressure (Po');

2) at a pressure close to Po', the curve becomes much steeper and a

curved portion exists, representing the transition between

reloading and new loads;

3) beyond the Po' point the curve is nearly linear (this linear portion

is termed the virgin compression curve and represents loading of

the sample at stresses greater than it has previously experienced);

4) if the pressure applied to a sample is reduced, a rebound curve

represents the readjustment of the sediment framework to the

reduction in load; and

5) if a sample is unloaded and reloaded, the curves from a hysteresis

loop.

These features can be observed on the e -log p' curves of Figures 3-3 to 3-8,

which show the data for this study.

74

Two parameters derived from the shape of the e - log p' curve are the

compression index (Cc) and the expansion index (Ce). Cc is the slope of the

virgin compression portion of the e - log p' curve and is calculated as

c, =!1e I log (P2IPI), (3-2)

where PI and P2 are any pressures on the straight-line portion of the virgin

compression curve and !1e is the void ratio difference corresponding to

pressures pz and P2. Ce is calculated in a similar fashion as the slope of a

straight line approximation of the rebound portion of the e -log p' curve.

The e - log p' curve can also be used to estimate the sample's

preconsolidation pressure (the greatest load to which a sediment has been

subjected; abbreviated Pc'). This parameter is defined by Taylor (1948) as the

"maximum past pressure" and represents the degree of consolidation which

the sample has undergone in the sediment column. A graphical method,

derived by Casagrande (1936), is often used to determine Pc'. In soil

mechanics terminology, a deposit is said to be normally consolidated if the

effective overburden pressure (Po') is equal to Pc'. Po' is the buoyant weight

of the overlying sediment, or the weight of the overlying sediment minus

the weight of the pore water. The effective overburden pressure of a

sediment at a depth z can be calculated as Po' = Y z, where y is the effective

unit weight of the sediment.

The ratio of Pc' to Po' is termed the overconsolidation ratio (OCR).

Generally, an OCR greater than one (Pc'> Po') indicates an overconsolidated

soil, i.e., the soil has been subjected to an effective pressure greater than the

present overburden pressure. An OCR of 1 (Pc' =Po') is considered to

75

indicate a normally consolidated soil, and an OCR less than one (Pc' < Po~)

indicates underconsolidation.

The shape of the e -log p' curve is strongly influenced by the amount

of disturbance that occurred during sampling, preservation, storage, and

specimen preparation, and also by the soil type. Increasing the degree of

disturbance flattens the curves considerably. Sandy or coarse-grained

sediments also result in a more rounded curve, with a more gradual

transition from reload to virgin loads, as found by several studies for several

types of coarse-grained sediments (e.g., Nacci et al., 1974; Demars, 1982; and

Marsters, 1986).

Schmertmann (1955) found that the laboratory virgin compression

line may be expected to intersect the in situ virgin compression line at a void

ratio of approximately 0.42% of eo (in situ void ratio). Schmertmann

suggested a method for the reconstruction of the in situ consolidation line

which would eliminate the effects of disturbance on the slope of the virgin

compression curve. Cc can then be calculated as the slope of this corrected

curve.

Procedures

Consolidation Tests

We performed consolidation tests on nineteen samples from the

Ontong Java Plateau. Samples were obtained as whole-round sections of core

from oozes at all five sites drilled. Samples 10 em in length, still encased in

core liners, were sealed with several layers of beeswax to prevent desiccation.

The wax-encased samples were kept in a refrigerator on the ship, and

76

submerged in water in a refrigerator in the shore-based laboratory after being

hand-carried from the ship.

We cut and trimmed each sample just before placing it in the

consolidation cell, to minimize moisture loss. A 6.2-cm inside diameter, 4­

cm high thin-walled ring with a sharpened cutting edge was pushed carefully

into the sediment sample encased by the liner. The sediment and embedded

ring were extruded from the liner, and the ends of the sample trimmed to a

smooth surface. We further trimmed the sample to approximately 2 em in

height, and then transferred it to the consolidation ring between two water­

saturated porous stones. The inside wall of the consolidation ring was

polished stainless steel, minimizing friction between the wall and sample.

Sample trimmings were used to measure water content of the sample.

Calculation of an initial void ratio (ej) was made using the water content

data measured from trimmings and the known volume and weight data of

the actual sample.

Head (1986) discusses the general techniques of laboratory

consolidation testing. Specific procedures undertaken in our consolidation

tests are described as follows. The tests were performed in back-pressured

consolidometers. The decrease in hydrostatic pressure as a sediment core is

brought to the sea surface may result in dissolved gases being released as

bubbles in the pore water. These bubbles are extremely compressible

compared to the relatively incompressible pore water and sediment grains,

and can thus severely influence the consolidation test data. Such bubbles can

also reduce the effective permeability of the sample. Compressed air was

used to apply back pressure simultaneously to the saturating fluid and

77

sample. While such pressures cannot duplicate in situ pressures for high

hydrostatic environments, e.g., for the Ontong Java Plateau, they are

sufficient to drive the gasses back into solution. We applied back pressure to

the samples at the rate of 10 kPa per half hour and then allowed a 24-hour

period at the final back pressure for the sample to reach pore-water

equilibrium prior to beginning the test.

Each load sequence doubled the previous load, such that load

increments were 4, 8, 16, 32, 64, 128, 256,512, 1024,2048, and 4096 kPa (the

maximum load provided by the equipment). Drainage was allowed from

both the top and bottom of the sample. Transducers, attached to the top

loading platten, transmitted sample height data to a data logger and

computer, where a file of time elapsed and sample height was created for

each load increment. The computer also provided continuous plots of

square root of time versus sample height so that the end of primary

consolidation could be estimated as the data were being collected. When the

end of primary consolidation was judged to have been reached, we applied

the next load increment. When the load and unload increments were

completed, we carefully removed the samples from the cells. Index

properties were measured on the consolidation sample to provide end-of-test

data for calculation of final void ratio.

Sample height versus square root elapsed time data were used to

determine the height (H100) corresponding to 100% primary consolidation,

using the method of Taylor (1948). From this H100 value, void ratio is

calculated as

(3-3)

78

where ei is the initial void ratio and H; is the height of solids. Hs is

calculated as

n, = WJGsApw, (3-4)

where Ws is the weight of solids (corrected for salt), Gs is the specific gravity

of solids, A is the cross-sectional area of the sample, and Pw is the density of

water. Void ratio versus log effective vertical stress (applied load minus back

pressure) was plotted, and the Casagrande (1936) construction was used to

determine the effective preconsolidation pressure, Pc'.

Other Analyses

We used trimmings from the consolidation samples to perform grain

size, microfossil counts, and scanning electron microscope (SEM) analyses.

SEM samples were oriented in marked plastic cubes, sealed in wax, placed in

a plastic bag with a small piece of wet sponge, and refrigerated. Samples

selected for SEM analyses were embedded with low-viscosity resin using

several stages of embedding designed to preserve the structure of the

samples. However, it should be noted that the embedding procedure was not

successful for many of the samples, as the sediment disaggregated during the

embedding process. These samples could not be used for SEM analysis.

Hardened embedded samples were cut and polished, and examined using a

back-scatter electron detector.

Analysis of microfossil content and preservation was performed on

four consolidation samples. Dry samples were wet sieved through a screen

with 63-llm openings and the resultant coarse and fine fractions examined

separately. The dry sand fraction was split by microsplitter to about 1/128 to

79

1/256 of the original amount and the particles, which numbered about 1000,

were identified and counted. In the analysis of the coarse fraction,

microfossils greater than 50% complete were counted as whole specimens.

Light microscope slides were prepared for the study of the silt and clay

fraction. Estimates of the percent frequency of the components of the fine

fraction were made from scanning most of the slide at 600 magnification.

Further details of the procedures for these analyses are given in Chapter 2.

SEM, grain size, and microfossil count data from the above-described

analysis were augmented by smear slide data from the core descriptions

(Kroenke et al., 1991) where appropriate. Due to the high frequency of small­

scale fluctuations in sediment properties (observed in grain size, carbonate

content, porosity, velocity, and other downhole data), we only used smear

slide data in cases where the smear slide depth and the consolidation sample

depth were within 10 em of each other. Laboratory methods for the

shipboard determination of carbonate and smear slide data are found in

Shipboard Scientific Party (1991b).

Results

e -log p: data for the 19 consolidation tests are presented in Figures 3-3

through 3-7; the data are organized by site (Site 803 through 807). Sample and

consolidation test data, including sample depths, effective overburden

pressures (Po'), effective preconsolidation pressures (Pc'), overconsolidation

ratios (OCR), compression indices (Cc), and expansion indices (Ce), are

summarized in Table 2-1.

80

The shape of the e -log p' curve is strongly influenced by the amount

of disturbance that occurred during sampling, preservation, storage, and

specimen preparation, and also by the soil type. Increasing the degree of

disturbance flattens the curves considerably. In addition, sandy or coarse­

grained sediments results in a more rounded curve, with a more gradual

change in slope between the reloading and virgin compression portions of

the curve. In these cases, a specific point of maximum curvature could not

be defined, and a minimum and maximum value were selected, resulting in

a range for Pc' for the sample. The method suggested by Schmertmann (1955)

was used to obtain an undisturbed virgin compression curve, and Cc was

calculated as the slope of this corrected curve.

Testing problems have limited the data available from some of the

tests, as described below. Testing errors resulted in no data for pressures less

than 100 kPa for samples 2 and 12. Because of the effect on the slope of the

virgin compression curve and the inability to estimate Pc' from the two e ­

log p , plots, neither Pc' nor Cc data are reported for these two tests. A

malfunction of the DCDT used in collection of height data occurred for the

first several load increments of Test 3, so that only two values were obtained

on the virgin compression line. The unload portion of these three tests,

however, are discussed later in this chapter, and are also used for calculation

of a rebound correction in Chapter 4.

Sample 7 was obtained from XCB-cored sediment and consists of stiff

light reddish-brown ooze (Kroenke et al., 1991). We found it difficult to cut

and trim this sample, and some patching of the sample ends was required.

The sample was tested despite these difficulties because it was obviously

81

different from the other samples; however the e -log p' cu.rve must be

considered to be a "disturbed" curve. Much higher values for Cc and Ce

were measured for this sample than for any other. Since the effect of

disturbance is to flatten the slope of the virgin compression curve, the value

obtained for Cc can be considered to be a lower limit of the possibly higher

"undisturbed" value. The high value obtained indicates that this sample is

much more compressible than the others.

Our descriptive analyses of the samples indicated them to consist of

white calcareous ooze, ranging from soft for the near-surface samples to stiff

for the samples near 200 mbsf. Sample 7 is the exception; we observed it to be

a light reddish-brown stiff ooze throughout its length.

The results of microfossil analysis performed on consolidation

samples 10, 11, 12, and 15 are presented in Table 2-2. The percentages of

coarse and fine fractions for the four samples are presented in Table 2-2a.

The breakdown of coarse particles as a percentage of the number of coarse

particles is presented in Table 2-2b, and Table 2-2c shows the fine fraction

constituents as a percentage of the fine fraction.

SEM photos for consolidation samples 2, 6, 11, and 12 are presented as

Figure 3-8. Some damage to the sediment fabric in these SEM samples is

observed to have occurred during embedding of the samples with resin.

Discussion

Consolidation Test Data for Carbonates

In general, the consolidation test results from this study are consistent

with those of other researchers of carbonate sediments (Morelock and Bryant,

82

1971; Demars, 1982). The values of compression indices (Cc) for our samples

are in the range 0.4 to 0.7 (with the exception of sample 7, with a value of

1.14). Demars (1982) obtained Cc values in the range of 0.35 to 0.65 for

sediments with carbonate contents of 60% to 90% from the Eastern Atlantic, a

few hundred miles off the coast of North Africa. Morelock and Bryant (1971)

obtained values of Cc in the range of 0.45 to 0.6 for sediments with carbonate

contents of 80% to 85 % from the West Florida continental slope.

Expansion index (Ce) values are also similar to those measured by

other authors. This study obtained values of 0.01 to 0.06. Demars (1982)

obtained values ranging from 0.04 to 0.11 (for carbonate contents of 60% to

90%) and Morelock and Bryant (1971) obtained values from 0.02 to 0.07, for

sediments with carbonate contents of 80% to 85%.

Pc' and Stress History

The majority of the samples tested appear to be overconsolidated, i.e.,

Pc' is greater than Po'. The definition of "normally consolidated," however,

takes into consideration only mechanical loading of the sediments.

Hamilton (1964), Bryant et al. (1967), and Richards and Hamilton (1967)

suggest factors that cause unusual structural strength will result in apparent

overconsolidation. These factors include cementation of sediment grains,

influence of secondary compression, age of the deposit and slow rates of

deposition. Many authors (e.g., Morelock and Bryant, 1971; Kelly et al., 1974;

and Rezak, 1974) discuss the role of carbonate cementation and other

physico-chemical bonding as the cause of apparent overconsolidation in

carbonate sediments.

83

However, Nacci et al. (1974) believe that the variation in OCR with

carbonate content is misleading. They found that the general change in

shape of the curves that occurs as the carbonate content increases results in

changes in the estimated preconsolidation pressure (Pc'), which are less

related to stress history and aging effects than to changes in the compression

mechanisms.

We found no relationship between OCR and CaC03 content or

foraminifer content. It is possible that the difficulty in determining P/ for

these high carbonate sediments results in poor estimates of Pc' and,

subsequently, of OCR. If this is the case, Cc would also likely be affected. The

problem of determining appropriate Cc and Pc' for sediments with high

CaC03 content (and for other sediments that result in the observed e -Iog p'

curve) bears further study.

Shape of Consolidation Curves and Intraparticle Water

The presence of intraparticle water in calcareous sediments and the

possible effect on consolidation behavior has been discussed by Nacci et al.

(1974), Demars (1982), and Valent et al. (1982). Choquette and Pray (1970)

define intraparticle porosity as porosity within individual particles or grains.

Interparticle porosity is defined as the porosity between particles.

The effect of intraparticle porosity on the physical properties of

calcareous sediments is generally dependent on whether the intraparticle

water is released to the system through grain fracture and crushing. For

example, release of intraparticle water during consolidation test loading

would significantly alter the shape of the e -log p' curve (Demars, 1982).

84

Most studies have reported no significant crushing of whole microfossils

during consolidation tests of carbonate samples (Bhattacharyya and

Friedman, 1979; Demars, 1982; Valent et al., 1982; Lind, 1993). Bhattacharyya

and Friedman (1979) and Valent et al. (1982) suggested that the fine-grained

fraction carries the loads applied and protects the shells of foraminifers

against crushing. Both studies showed that a sediment sample containing a

greater percentage of intact foraminifers, Le., containing less percentage of

load-carrying matrix, exhibited more grain chipping, fracture, and crushing

during consolidation tests.

Evidence of grain crushing and release of intraparticle water that

would be expected in consolidation test curves would be virgin compression

curves that are concave upward and to the right. The release of intraparticle

water with grain breakage would temporarily but noticeably reduce the

strength of the sediment. Once the sample is compressed under increasing

loads, the resistance to deformation would increase and the curve would

become less steep (assuming, of course, that continuous release of

intraparticle water did not occur).

The consolidation test curves for our study do not show any

indication that breakage of foraminifer tests and release of intraparticle water

has occurred. Microfossil and SEM data provide estimates of the relative

percentages of intact foraminifers for some of our consolidation samples.

We see no difference in the general shape of the virgin compression portion

of the curve for sediments with lower or higher percentages of intact

foraminifers, indicating that these tests are not being broken at the pressures

used. SEM analysis of 4 samples taken from the consolidometers after testing

85

was completed also show no evidence of significant grain fracture or

crushing (Figure 3-8). This phenomenon is also observed for samples

obtained from much deeper in the sediment column (Lind, 1993). SEM

analysis of chalks, recovered from greater than 300 mbsf, show large numbers

of foraminifers still intact (Wilkens, 1991).

Correlations with Microfossil Content

As previously discussed, many authors have found correlations

between specific parameters and CaC03 content. This relationship between

sediment composition and consolidation characteristics is emphasized by the

behavior of Sample 7. The sample is described as a light reddish-brown ooze

and termed as a radiolarite by shipboard sedimentologists (Kroenke et al.,

1991). Shipboard smear slide analyses estimated the composition at this

depth (202 mbsf) as 30 to 40% radiolarians. Shipboard carbonate content

analyses indicated a minimum calcium carbonate content of 65-75% at this

depth. Sample 7 exhibits much higher compression and expansion indices

than any other sample; these properties are clearly related to the increased

siliceous fraction of this sample. Siliceous sediments have been shown to

exhibit high initial void ratios and compression indices greater than 2 (Lee et

al.,1990).

We have plotted compression index and expansion index versus

carbonate content in Figure 3-9. The data indicate that values for both

parameters decrease with increasing CaC03 content. This relationship

corresponds with that observed by other authors for sediments with a wide

range of carbonate contents (Morelock & Bryant, 1971; Demars, 1982;Silva

86

and Jordan, 1984; Rack et al., 1993). Compression indices have been shown to

be greater than 2 for siliceous sediments (Lee et al., 1990) and greater than 6

for clay sediments (Fukue et al., 1986;Crawford, 1986).

We also plotted compression index and expansion index versus

percentage of foraminifers (Figure 3-10). Although the compression index

data for which we have foraminifer counts are limited, a trend of decreasing

Cc with increasing foraminifer content can be discerned. More expansion

data are available, and the trend of decreasing Ce with increasing foraminifer

content is quite evident. This relationship between consolidation

parameters and microfossil content (or microfossil grain size) have been

observed in previous studies (Keller & Bennett, 1970; Hamilton, 1974; Lavoie

and Bryant, 1993). The observed relationships between consolidation

parameters and microfossil content are probably related to the volume of

compressible matrix. As previously discussed, evidence regarding grain

crushing indicates that the fine-grained fraction carries the applied loads. An

increase in the relative percentages of incompressible grains (e.g., intact

foraminifers or sand grains) would result in less compressible material and,

subsequently, lower c, and Ceo

Conclusions

Our study of the consolidation behavior and the relationship of

consolidation parameters to microfossil content for the high-carbonate

sediments from the Ontong Java Plateau has resulted in the following

conclusions:

87

1) In general, consolidation test parameters from this study, namely

compression and expansion indices, are consistent with those of

other researchers for sediments of similar carbonate contents;

2) The difficulty in determining accurate Pc' values for these granular

carbonate sediments precludes determining any relationships

between overconsolidation ratio and carbonate content or

microfossil content;

3) There were no effects of intraparticle water observed in the shape

of the consolidation curves, indicating that microfossil tests were

not broken and significant release of intraparticle water did not

occur during consolidation testing. This conclusion is supported by

the lack of foraminifer test crushing or breakage observed in the

SEM micrographs; and

4) Correlations between both compression and expansion indices and

carbonate and foraminifer contents were found. We found that

both parameters decrease with increasing carbonate content; this

relationship is linked to lower compressibility of a carbonate

sediment matrix relative to siliceous or clay sediments and is

consistent with the findings of other studies. The two

consolidation indices were also found to decrease with increasing

foraminifer content. This relationship is likely caused by a lower

relative volume of compressible matrix with increasing

percentages of relatively incompressible grains.

88

Table 3-1. Consolidation Test Results

Coefficient of Compression ExpansionTest # Site Depth Po' Pc' OCR Consolidation Index Index

(mbsf) (kPa) (kPa) (em Is)

1 803D 14.93 74 490 6.6 0.016 0.70 0.052

2 803D 118.51 656 -- -- -- -- 0.022

3 803D 212.03 1194 -- -- -- -- 0.038

4 804C 4.22 21 190-280 9.0-13.3 0.015 0.71 0.051

5 804A 14.92 76 130 1.7 0.020 0.66 0.042

6 804C 99.22 525 1300 2.5 0.032 0.47 0.045

7 804C 202.23 1139 805 0.7 0.010 1.14 0.06600 8 80SC 15.23 75 510 6.8 0.016 0.68 0.036\0

9 80SA 26.18 129 210 1.6 0.016 0.59 0.047

10 BOSC 31.22 154 305 2.0 0.017 0.59 0.038

11 805B 98.13 524 260 0.5 0.015 0.48 0.017

12 805B 204.12 1129 -- -- -- -- 0.055

13 806B 18.94 96 280-400 2.9-4.2 0.013 0.67 0.036

14 B06A 32.43 164 350 2.1 0.021 0.53 0.015

15 8068 118.41 620 4000 6.5 0.028 0.39 0.021

16 806B 211.91 1172 510-900 0.4-0.8 0.024 0.72 0.033

17 807A 14.84 73 260 3.6 0.034 0.64 0.049

18 807A 100.34 526 350-570 0.7-1.1 0.030 0.68 0.035

19 807A 206.34 1162 830-1050 0.7-0.9 0.037 0.36 0.011

Depth(rnbsf)

SiteTest J.D.

Table 3-2a. Grain size data for four consolidation samples expressed as weight percentage of total sample.

Coarse fraction Fine fraction(% total (% total

samE!~eightL_ _~~mple weight)

10111215

805B805B80SC806B

31.2298.13204.12118.41

19.406.493.62

29.62

80.6093.5196.3870.38

Table 3-2b. Coarse fraction constituents for four consolidation samples; constituents expressed as percentage of total coarse particles.

Planktonic Benthic ForaminiferTest J.D. Site Depth foraminifers foraminifers fragments Radiolarians Diatoms Other Minerals

(mbsf) ('Yo coarse) (% coarse) (% coarse) (% coarse) (% coarse) (% coarse) (% coarse)

10 805B 31.22 32.4 0.5 57.5 9.5 0 0 0.1~ 11 805B 98.13 28.4 0.5 56.8 14.1 0 0 0.3

12 80SC 204.12 23.6 0 32.4 43.5 0.2 0.2 0.215 806B 118.41 44.8 0.6 48.3 6.0 0 0 0.3

Table 3-2c. Fine fraction constituents for Hole 803Djconstituents expressed as a percentage of total fine particles.

Nanno- Foraminifer Unidentified Radio- Organic Unidentified UnidentifiedTest J.D. Site Depth fossils fragments calcareous larians Diatoms carbon minerals silicious Preser-

(mbsf) (%) (%) particles (%) (%) (%) (%) (%) particles (%) vation

10 805B 31.22 86 4 7 1 1 0 <1 <1 good

11 805B 98.13 85 2 6 2 3 <1 <1 1 moderate

12 805C 204.12 87 5 5 1 1 <1 <1 <1 mod/good

15 806B 118.41 86 10 3 <1 <1 0 <1 <1 good

Figure 3-1. Calcium carbonate data for five site drilled during ODP Leg 130.

91

~VJ

,.0.sN..r::oW

g-o

Site 803csco, (%)

50 60 70 80 90 10001 ill I 'I ill! I I I.! I.m! ill Ii ! i·it~·: : : .

I i r· .j~ 1· 1-.'~

1001- .. ··~·· .... ·i·· ....·j ..····~·; : ; ·"1·i II¥! i i .~: 1 1 ~

2001-....·:· ..· .. \· ..·"j"?'~; : : ,."J;••l 1 j .'• •~.: : : ~ .: : : : "'.: : : ": : : .-; ..1 1 j .~ ..'

300~··· .j ! j..J.: : : : ..

400~····,·······!········I···.::r·: : : :..:: : : :' .: : : :.i ! ! ~

5001-··I··! ..·j"·i{''.1 1 1 "1:.: : :':': : :":: : 0: ~

j ~ l' ~: : : :.

.• 1 •••• 1 •••• 1

Site 804csco, (%)

50 60 70 80 90 100II Ii I , I iI I I " iU t.... I' I I • iI

.J.....··I· ..l~;t~a~~:

. .t·!

.' -,' ~'" ;

•• 0 ••• t. i, .:: 'i. .:

:' 1II

....~'l J..J~.! '., I

" ·11··

........ ;f'. .i

Site 805csco, (%)

50 60 70 80 90 100I" I.! I. iI!' "' "! "&~ "'iI

! ! i)~! i i ~l'. . .: : ! •: ; ; ·:t.

.J.....~,~~..-.:'~

.~-·11..: : :,.,

I I I ~~lt:.

1······j········I··~i; ; i I"

111>~! I l..t!~ l ~ ~•.iiii t~

. t J i ~~..-Iii 'VI I I ·f:I I I ·t:;

Site 806csco, (%)

50 60 70 80 90 100f'T'"'TT'"'T"'"' I 'I'V~I' i. i

iii ::~.: ; : c

t· . .· . .· . .! I !"l~· . .

·..·..i···· ..··i......··i· ..·~·f ..-:!J

~it~ ~ ~ S':.; : ; i~

...... j· ....···f·....···l· ..·..··j·1l-: : : : -'i

I I I I~': : : =1

······1······1········1·······~-i ! i l·t~ ! ! !~, , : :.!\.

. i l. ~ I.:.l-

I I I i~i ~ i 1;:

111';1l~ ~ ~ r~:! i ~ ~--

i i .L ... L:

Site 807csco, (%)

50 60 70 80 90 100I" " I' i , , IIi iii Ii Y:I: i.,

; : : ,.! ~ i -,\iii ';If

1111·iii !\{· . . .· . . .· . . .

Iflll-i I I ~,i i ! ! S.: : : :i

L.... ..l.~1 ~;..5!: ! -i ~..: I..l ..i.', .:~

I:~~~, ,,...i r-.j~ j .:l i'Ji i.• ·~i i "I,

. ..\i l .~

i i 1; ;.)-l t-.-...

Figure 3-2. Schematic showing the relative water depths of the five sites

drilled during ODP Leg 130 (taken from Kroenke et al., 1991).

93

Site 8062520m

0

100

~ 5Ma200

e;:-

II 300E--- i 10 Ma.ca. 400Q)

0

500 ~ 15 Ma

\0~ 600 ~ 20 Ma

700

- Seq!I.-_OOr

OO<>e - Site 805- 318Bm

o Ooze

[Z2j Ooze with stilf intervals

I:sJ Ooze with chaik intervals

DChalk

- _ Site 804- 3861m

- ~

Figure 3-3. Void ratio versus effective vertical stress (e -log p~) for Samples

1, 2, and 3 from Site 803. The numbers in the legend are the

depth in meters below seafloor for each sample.

95

10000

e·.,

,,,•,,

e··. 14.93

~ 118.51

+ • .;- 212.03

,,•,,,

tt

.. ,..

10 100 1000Effective vertical stress (kPa)

e· . ........ ...

,8........ .,

...... -e. .. '......... \..... -e .. - • ... -e

2.2

2.1

2.0

1.9

1.8

1.7

1.6

0 1.5 .'';:;

~"'0 1.4'0> 1.3

1.2

1.1

1.0

0.9

0.8

0.71

96

Figure 3-4. Void ratio versus effective vertical stress (e -log p') for Samples

4,5,6, and 7 from Site 804. The numbers in the legend are the

depth in meters below seafloor for each sample.

97

10000

e- -. 4.22~ 14.92

+- -+ 99.22

... - ... 202.23<,.... ." o.... o.

."" .. o.eO.

10 100 1000Effective vertical stress (kPa)

+- ..---.- ~- ..---.- ""i- .. -.,.. 4~- -----~ -. ,

----.- \---"'---..+

..... -tit.

' ..o.

o...,..~

\ ...., ..\ .., .

8, .', .... ...... \~ rr «; - ' '.""+-..:.,.. - • • .. .... \ ..

.......".- - .. -. 1..."-I-- ~l' .....

\,..'

2.2

2.1

2.0

1.9

1.8

1.7

1.6

0 1.5.+::

~1.4'"C

·0> 1.3

1.2

1.1

1.0

0.9

0.8

0.71

98

Figure 3-5. Void ratio versus effective vertical stress (e -log p') for Samples

8, 9, 10, 11, and 12 from Site 805. The numbers in the legend are

the depth in meters below seafloor for each sample.

99

.- -. 15.231::.- ~ 26.18

-1--";" 31.22

...-.-. 98.13

*--K204.12

2.2

2.1

2.0

1.9

1.8

1.7

1.6

o 1.5.~

"'C 1.4'0> 1.3

100 100010 . I stress (kPa)Effective vertica

100

Figure 3-6. Void ratio versus effective vertical stress (e -log p') for Samples

13, 14, 15, and 16 from Site 806. The numbers in the legend are

the depth in meters below seafloor for each sample.

101

.·-e 18.94l!r---A 32.43

+- --+- 1 1 8 •4 1

'--"'211.91

.. -~ .l.~_ -2- _-+-- ~ ....................~..........

i ----1.. _ _ -L _4--!--- - ---1-- - - -r ,

' \

\ ,\ ,.---...._--......_---..._-.......-- ....

.... - .... -+-- ... - .....

-....-

....... ............ .. .....

1

2.2

2.1

2.0

1.9

1.8

1.7

1.6

1.2

1.1

1.0

0.9

0.8

0.7

o 1.5.~

~ 4-c 1.·0> 1.3

102

Figure 3-7. Void ratio versus effective vertical stress (e -log p') for Samples

17, 18, and 19 from Site 807. The numbers in the legend are the

depth in meters below seafloor for each sample.

103

•••• 14.81

~100.34

+--+ 206.34

10 100 1000 10000Effective vertical stress (kPa)

.. • • • • .. =fot •••• - -•• - - - - • - - -""c. - •,

~---+ -I- ~--- ----I----+--..;::lt-

+-----+_-1......,...-~

-""::pt-

'""+- '

2.2 ..-------------------,

2.1

2.0

1.9

1.8

1.6

0 1.5.';:

e1.4"'C

·0> 1.3

1.2

1.1

1.0

0.9

0.8

0.71

104

Figure 3-8. SEM images of consolidation samples. Image A is the 99.22

mbsf sample from Hole 804C (consolidation test #6). Image B is

the 204.12 mbsf sample from Hole 80SB (consolidation test #12).

Image C is the 118.51 mbsf sample from Hole 803D

(consolidation test #2). Image D is the 98.13 mbsf sample from

Hole 80SB (consolidation test #11).

105

106

Figure 3-9. Compression index (Cc) and expansion index (Ce) versus

carbonate content.

107

100 r-T'"T""T'" .,..,. ,......, ..-..-.-..-.-...-.- ..,...,...,...,. ........

•

-?fl.-~ i·. .90 -. ; .I · .---.---.--'-----.~--.---

00 ·----~r--~·-I---·---·!-··------- .0.4 0.6 0.8 1 1.2

Compression Index, Cc

100 r--............---,.-.....--r--.----,.-.....--r-...,...---,.-.....-......-.----,.-.....-......-.----,.--,

90-?fl.­..,8a 80

0.02 0.04

Expansion Index, Ce

108

0.06 0.08

Figure 3-10. Compression index (Cd and expansion index (Ce) versus

foraminifer content.

109

20 1"""""1"""I'"",..,........-.,."T""T..........,....,...,....,~~,..,...,r-T""r-T""I"""I'""....,.. .........,....,."T""T..........,....,~"'"T""'l,..,...,.-.-,...,...,

10 .

!' .5·_·-·_·-~----r--·--

O~......",.......",. ...L...l:I ~

0.2 0.4 0.6 0.8 1.2

Compression Index, Cc

0.080.040.02

·············T~······································· ; j •••••••••••••••••••••••••••••••••••••••••••

10 ; j ••••••••••• .. i·· ..·· · · ·········· ..·····

• , ! '.

: :5--:----r~-··················,····················---

-cfl. 15 .--c~-ceUlU...~'c'slU...

&::

Expansion Index, Ce

110

CHAPTER 4

POROSITY REBOUND OF ONTONG JAVA PLATEAU SEDIMENTS

Introduction

The essential problem in relating laboratory measurement of density

and porosity on samples from a borehole to in situ values is to determine or

estimate the increase in volume (rebound) resulting from the removal of the

sample from the pressures of the overlying sediments. Rebound of a

sediment occurs when the in situ overburden pressure is removed. The

correction of laboratory porosity data for rebound allows an approximation of

in situ porosities. In situ porosity (and in situ density) data are frequently

used to calculate sedimentation rates and for construction of synthetic

seismograms; therefore correction of shipboard porosity can be essential

where downhole logging data are missing or erroneous.

Hamilton (1976) presented a method, based on examining the

unloading curves of consolidation tests, for correcting laboratory porosity

data for rebound. Hamilton's (1976) carbonate model included all sediments

with a calcium carbonate content greater than 30%, and his generalized

downhole laboratory porosity profile is presented in Figure 4-1, along with

shipboard-measured porosities for site 806 from the Ontong Java Plateau

(Kroenke et al., 1991). The Ontong Java Plateau sediments, with calcium

carbonate contents generally near 90% (see Figure 3-1 in Chapter 3), exhibit

much different physical behavior.

Shipboard efforts to correct laboratory porosity data, using Hamilton's

rebound correction for carbonates, indicate that Hamilton's model is not

applicable to the Ontong Java Plateau sediments. Shipboard laboratory

111

porosity data and in situ porosity data (calculated from downhole density log

data) for Hole 803D are plotted in Figure 4-2. The laboratory porosity data

corrected using Hamilton's rebound model are also shown in Figure 4-2.

The Hamilton correction clearly overcompensates for the rebound of the

Ontong Java Plateau sediments. We have used rebound data from

consolidation tests performed on Ontong Java Plateau sediments to correct

shipboard laboratory data to approximate in-situ conditions.

Shipboard Procedures

The methods used to measure porosity in shipboard laboratory and

downhole logging programs are detailed in Kroenke, Berger, Janecek et al.

(1991). In general, we used one or two samples per 1.5-m section of core for

the laboratory determination of index properties. Wet samples were

weighed to ± a.01g using a motion-compensating balance and placed in an

oven at 1l0·C for 24 hr. Dry samples were removed from the oven, placed

in a desiccator, and also weighed to ± O.01g. Dry volumes were measured to

± 0.02 cm3 using a helium-displacement pycnometer. Porosity is the ratio of

the volume of voids to the total sample volume, expressed as a percentage.

Porosity (<I» was calculated from the downhole logging bulk density

data, using the equation:

(4-1)

where Pb = sediment bulk density, Pg = sediment grain density (2.72 g/cm3

assumed), and Pw =density of sea water (with in-situ temperature and

pressure effects accounted for).

112

Rebound Model from Consolidation Data

We have used the unload portions of the void ratio vs, log effective

stress (e-log p') curves from the 19 consolidation tests described in Chapter 3

to estimate the elastic rebound (increase in volume) that occurs when the

stress applied to the sample during the test is removed. The consolidation

tests were performed on samples from all five sites drilled during Leg 130,

and the sample data were presented in Table 3-1 of Chapter 3. The rebound

portion of an e-Iog p' curve results from the removal of pressure from the

sample. The assumption in utilizing these rebound data to correct laboratory

porosity to approximate in situ conditions is that the laboratory rebound of a

consolidation sample is analogous to removal of sediment samples from the

overburden pressures of a borehole. This assumption appears valid, because

several authors (Schmertmann, 1955; Hamilton, 1976) and this study

(Chapter 3) have noted that rebound curves for a particular sample are more

or less parallel regardless of sample disturbance or pressure from which

rebound started. Schmertmann (1955) discussed the probability that geologic

rebound (removal of overburden due to erosion), if similar plots could be

made, would also be parallel to laboratory test rebound.

It was assumed that the value of void ratio at atmospheric pressure is

insignificantly different from the value shown on the rebound curves of the

e-Iogp' plots at 1 kPa pressure. The rebound curve was extrapolated back to

1 kPa from the last unload increment (4 kPa for most tests); the curve

connecting the last several data points of the unload section is linear for

most of the tests (see e--Iog p' curves in Chapter 3). We then converted the

void ratio corresponding to the maximum stress applied during the

consolidation test and the void ratio at 1 kPa to fractional porosity values

113

using n =e / (1 + e). We calculated the porosity rebound as the difference in

these two values, expressed as a percentage. Porosity rebound from pressures

indicated to laboratory pressure for the 19 samples tested is shown in Figure

4-3. The regression equation is:

rebound =8.1E-04 P - 5.4E-08 p2, (4-2)

where P is pressure in kPa. The equation is constrained to zero rebound at

zero pressure.

Overburden pressure which causes sediments to decrease in volume

with increasing depth below seafloor is created only by the submerged weight

of the mineral grains. Overburden pressure is independent of depth of water

if free movement of water can take place so that water pressures in sediment

pore spaces can become hydrostatic. We calculated the effective unit weight

corresponding to the maximum stress for each test from the void ratio

obtained. We averaged this effective unit weight with a value of effective

unit weight calculated for the sediment at the top of the hole. We then

calculated the equivalent depth by dividing the maximum stress by this

average effective unit weight. Porosity rebound vs. equivalent depth is

presented in Figure 4-4. The regression equation is:

rebound = 5.4E-03 D - 2.5E-Q6 D2, (4-3)

where D is depth in meters below seafloor (mbsf),

The data indicate that the elastic rebound for these sediments is in the

1% to 4% range, for equivalent depths ranging from 200 to 1200 mbsf. These

values of porosity rebound are lower than those observed by Hamilton

(1976), who measured rebounds of 5% at an equivalent depth of 400 mbsf.

114

This difference in rebound behavior for the different sediment types gives

rise to the difficulty in using Hamilton's rebound correction equation for

Ontong Java Plateau sediments.

Examination of Figures 4-3 and 4-4 shows that Sample 7 appears to be a

higher-porosity outlier, exhibiting a rebound of 6%. This sample, from Hole

804C, was described as a light reddish-brown radiolarian ooze by shipboard

sedimentologists (Shipboard Scientific Party, 1991a). Shipboard smear slide

analyses estimate the composition at this depth as approximately 40%

radiolarians and approximately 60% nannofossils. The sample exhibits

considerably different consolidation and rebound characteristics than the

other samples at site 804 or other sites, as described in Chapter 3. Since the

major sediment type drilled during Leg 130 was white calcareous ooze, we

have omitted this sample from the calculation of a new curve fit (shown as

the heavier line in Figure 4-4) to provide the most reasonable rebound

correction for the majority of the Ontong Java Plateau sediments. The

equation then becomes

rebound =4.7E-Q3 D - 2.0E-Q6 D2. (4-4)

Correction of Shipboard Porosity Data

The correction of laboratory porosity data for rebound allows an

approximation of in situ porosities for zones where downhole logging data

are missing or erroneous. Downhole logging data are not available for all

sites or for at least the upper 100 m in logged holes. In Hole 803D, for

example, the downhole logging data are not useful above approximately 220

mbsf, due to a problem with the logging tool (Kroenke et al., 1991). The

115

laboratory and downhole log porosity data vs. depth below seafloor for Hole

803D were shown in Figure 4-2. There is a systemic difference between the

two data sets, characterized by higher laboratory porosities. This behavior

was observed for all sites and is attributed to the effects of removing the

laboratory samples from their in-situ positions. We have assumed downhole

logging data to approximate in-situ data, although there may be errors in the

downhole logging process and its interpretation.

Sediment rebound is a complicated phenomenon that we believe is

controlled by the contributions of two mechanisms. When a sediment

sample is removed from the total overburden stress state corresponding to

its in-situ position, an expansion of the sediment framework occurs due to

the decrease in the overburden stress state. The expansion that occurs during

retrieval of the core will depend on the sediment characteristics and the

length of the drainage path, and could be less than the maximum possible

expansion of the sediment. The second mechanism for porosity rebound is

pore fluid volume expansion caused by the release of hydrostatic pressure.

The expansion of the pore fluid is due to a change in seawater density and is

related in magnitude to the water depth (i.e., pressure) and the temperature

gradient. The increase of pore-water volume calculated for the water depths

for the sediments cored on the Ontong Java Plateau is in the range of 1% to

2%.

It is difficult to quantify how the two processes, namely (1) adjustment

of the sediment framework to the removal of overburden and (2) pore-water

expansion, contribute to the total porosity change. The response of the

sediment framework due to the removal of overburden pressure is

constrained to zero rebound at the seafloor. However, there is a small

116

increase in porosity of even the surface sediments upon removal from in­

situ conditions because of the expansion of the pore fluids related to the

change in seawater density.

We believe that rebound may be controlled by different mechanisms

according to depth below seafloor. For sediments near the sediment/water

interface, porosity rebound is controlled by pore-water expansion, resulting

in 1% to 2% change in porosity, depending upon water depth. As depth

below seafloor increases, the effect of overburden on the sediment increases,

and the associated resulting elastic rebound when that overburden is

removed also increases. It is considered unnecessary that the porosity change

ascribed to each of these processes should be added to correct the laboratory

data. At least part of the pore fluid volume expansion is likely

accommodated by the elastic rebound due to the removal from overburden

that occurs concurrently. The pore fluid expansion from release of

hydrostatic pressure may also release residual effective stress maintained

within the sediment, resulting in a lowered tendency of the sediment

framework to expand along the rebound curve.

We emphasize that elastic rebound due to the removal of overburden

pressure is not solely the result of seawater density decrease and associated

pore fluid volume increase. The data for these carbonate oozes from

consolidation test results indicate that the elastic rebound with depth

(Figures 2 and 3) is up to 4% and close to the amount of pore-water

expansion caused by decrease in pore-water density. However, for other

sediment types, such as silicious sediments (Lee et al., 1990; Marsters and

Christian, 1990) and clays (Hamilton, 1976), the elastic rebound can reach up

to about 10% and far exceeds what could be accounted for by expansion of

117

pore waters. Expansion of pore waters is dependent upon water depth and

thermal gradient and is not influenced by sediment type or constituents. H

pore-water expansion were the only mechanism responsible for porosity

change in sediments upon removal from pressure, no variation in rebound

would occur with varying sediment type.

Corrected laboratory porosity data vs. depth for the upper 150 m of

Hole 8030 are presented in Figure 4-5. For clarity, we have used a solid line

to show the data corrected for rebound, using the porosity rebound vs. depth

relationship of Equation 4-4. The points are the data corrected only for

expansion of pore waters, using the methods detailed in Urmos et al. (1993).

Because measured laboratory porosities are greater than downhole log or in­

situ values (Figure 4-2), the corrections involve a decrease in porosity. Near

the seafloor, the correction for seawater density is greater than that for

rebound, i.e., the correction for pore water expansion results in lower

porosities than the correction for rebound. With increasing overburden, the

correction for rebound increases until the two corrections merge (at a depth

between 100.7 and 123.2 mbsf for Hole 8030). Below this merge depth, the

correction for rebound becomes increasingly more significant with depth.

We have combined the two corrections into one by using the correction for

pore-water expansion in the upper sediments where it is more significant

than the rebound correction, and by using the rebound correction below the

midpoint of the depth range over which the two corrections merge.

Downhole log porosity and corrected laboratory porosity (corrected

using the rebound combination) vs. depth for the interval between 200 and

600 mbsf in Hole 803D are shown in Figure 4-6. The corrected laboratory data

provide a good fit to the downhole log data, with the exception of the depth

118

interval between 200 and 220 mbsf. Problems with the malfunction of the

downhole logging tool resulted in no data or erroneous downhole logging

data in the upper 220 m in this hole.

Similar downhole profiles of downhole log porosity and corrected

laboratory porosity for sediments from Hole 806B are shown in Figure 4-7.

Once again, the corrected laboratory data provide a good fit to the downhole

log data for depths greater than about 350 mbsf. Shipboard scientists (1991c)

reported the downhole logging data for the depth interval of 190 to 320 mbsf

to be unreliable.

As is evident from the above comparisons of downhole logging data

and corrected laboratory data, the rebound-corrected laboratory data can be

used in place of the downhole logging data when the logging data are

missing or erroneous. The combined seawater density/consolidation

rebound correction was used to correct shipboard porosities for use in

constructing synthetic seismograms (Mosher et al., 1993). A good match

between the synthetic seismograms and the field seismic record resulted

from the use of this correction.

Conclusions

The analyses of the consolidation behavior of calcareous sediments

from the Ontong Java Plateau have resulted in the following conclusions:

1. Rebound curves from consolidation tests on Ontong Java Plateau

samples yield porosity rebounds (resulting from release of effective

overburden stress) of 1% to 4% for these sediments (except one

radiolarian-rich sample) at equivalent depths of 200 to 1200 mbsf.

These values of rebound are significantly lower than those

119

reported for other sediment types, such as siliceous sediments and

clays, with reported rebounds up to about 10%.

2. A radiolarian-rich sample exhibits 6% rebound, which is consistent

with the higher porosity rebounds reported for siliceous sediments,

and illustrates the influence of sediment composition on porosity

rebound.

3. A rebound correction derived from the porosity rebound from

effective stress release has been combined with a correction for

pore-water volume expansion to correct the shipboard laboratory

porosity data to in-situ values. Comparison of the laboratory

porosity data corrected in this manner with the downhole log data

shows good agreement.

4. This rebound correction provides a more accurate correction for

Ontong Java Plateau sediments than Hamilton's (1976) carbonate

rebound correction, which is limited in its depth applicability and

includes sediments containing as little as 30% carbonate in the

model. Hamilton's correction is not applicable to Ontong Java

Plateau sediments, which generally contain 85% to 95% calcium

carbonate and exhibit markedly different rebound behavior. Based

on the observed variability in rebound for different sediments,

rebound corrections would be ideally performed only when

rebound data is available for a particular sediment type. However,

the rebound correction derived in this study for Ontong Java

Plateau sediments could likely be used for other high-carbonate

sediments.

120

Figure 4-1. Merged porosity (Holes 806B and 806C) vs. depth for Site 806

and generalized laboratory curve for calcareous sediments

(Hamilton, 1976), taken from Shipboard Scientific Party (1991c).

121

Generalized laboratorycurve for calcareous sediments(Hamilton, 1976)

\200

--en..0E- 400J::aCDo

600

Site 806 data(merged files)

757055 60 65Porosity (%)

50800 a......--'-_....-.----l_--J..._....I...----I._.....I.-_'"----'-_......----l_.....I

45

122

Figure 4-2. Hole 803D calculated downhole logging porosity (solid line) and

uncorrected shipboard laboratory porosity (dots). The shipboard

laboratory data corrected using Hamilton's model (crosses) are

also shown.

123

7065I I

Hole 803D Porosity (%)

50 55 6045. I . '":~ ~:X~~~

xo~~~~lx~Xa·.-~,;... ~ ,

i : i : ~_ ~t,~~~ •Ye e: : : : ~ jfr ,,- :

........................+ L \ ;( ~..~.~!...•.............+ .

! : x~:~x~••~• i: x x : .~:.: xX:· x·.:! x ~ •• lC~:.: : : X :.XI< xtl'e,.i : : x~ .~. :: : i 14 xxx e.. . ij i x! ><X • "xX •• :, : x: x~..~.· :........................1""" [ x, ~;..~x····~·~x"!~·..;··············~·· ..················_---: . x ~ : :i x~ ; !! xix ~ •

a40

200

300

100

­.....ell..ce-

x•

400

500

x ;.x

600

124

Figure 4-3. Rebound in % porosity change from pressures indicated to

laboratory pressure for the 19 consolidation samples. The fit to

the data has the equation: rebound =8.1E-Q4 P - 5.4E-08p2,

where P is pressure in kPa.

125

Porosity Rebound (change in %)

84321

: :

....L....•(~.~~~~ +.._ j + L. 1.. .

• •• ' Rebound e sr4P-5.'E~ ~!

.----- ....._- _·_·_--,·-····--r··_··_··r-···-·····__·····-······_···_·

: 807A:<100.34)• •804A "'"······"("14:92,..·················· ~ ······80'...:\····-;-···············..····;······················..·····················T·····················r···· .i . (14.84)i: ::i . : 806A : ::

I I . (;.43) : j:

__..L_.__.j..• _.....t.~--l--..L._ .._.._._.-A \: i 805A i : i

B06B ! ! !(26.18): : :(18.94): 804C : : : :

: (4.22) , :...................T ; ······· ·..·-;- : l.. ~ ; .

: 803D: (212.03)

0: •807A i ' ,

(206.34): • : :: S06B,..: i : (211.9~) i 804C :..·....········....·!805it··....··~O·5c..·....·· ·r·....·........·....···j....·........·..·....-r..··....··......(10Z;2jr..·····..T....··........·......(98.13) (31.22) :: :

: •• ~803D :805C :(14.93)

: (lr·23)

::::...................., ; ·· ·· ··..····;.S{)4€..·····-;-·······..·· ·· ·;···· · ; ..

: S06B : : (99.22) : : :

I(llS~:~o e est'(l1S.51) (204.12)

7000

6000

2000

1000

3000

-lU~~-~... 40000

<IJ<IJ~...=...

5000

126

Figure 4-4. Rebound in % porosity change versus depth (calculated from

pressures) for the 19 consolidation samples. The light solid line

is the fit to all 19 data points and has the equation: rebound =

5.4E-03 D - 2.5E-06 D2, where D is depth in meters. The heavy

line is the fit to the data excluding sample 7 and has the

equation: rebound = 4.1£-03 D - 2.0E-06 D2. Hamilton's (1976)

rebound curve for carbonates is also shown.

127

71

• x

ex

• x

ex. .>fIx

ox

e ex1tx

Ox

xC»

•ex

Porosity (%)

o

.-

xe

..~ r .-;;:"_:\_.:!_ l__'_xe . .. :::.: .·····················t························

•

150 __~_L_.r-_G ..1-_i.._.1-_l_...J..._-L_-L._J

125

100

­.....en,.Q

e-

128

Figure 4-5. Hole 803D shipboard laboratory porosity data corrected for

rebound (solid line) and for seawater density (dots).

129

Porosity Rebound (change in %)

S03D- (14.93)

CI 804C(99.22)

Rebound = 4.7E-03 D - 2.0E-06 D2

805B• (204.12)

32

SOsC.(15.23)

806B(118.41)-

803D(118.51) •

1

80sB •(98.13) SOsC

(31.22) •1000

1100

... ... ... ... --.100 ... ... ... ...

" "200 " " " " Hamilton rebound curve- "tI)

... /~ 300:::l '\tI)tI)

\culo< ...Q.,

e 400 ...Q 807A \lo< (100.34 \.....

"0 • • \cu

500 804A \-lo<cu (14.92) I> IC S06A IQu (32.43) I~ 600 / ,

CI.l,.Q ,e • .80sA I- (26.18).c: ,- 700 •Q., S04Ccu (4.22)Q"0C

800:::lQ e 806B1l 807A •

(211.91)0c=:: (206.34) 8030 804C

900 (212.03) (202.23)fa

130

Figure 4-6. Hole 803D downhole log porosity (solid line) and shipboard

laboratory porosity corrected using combined rebound and

seawater-density correction (dots).

131

Porosity (%)

40 45 50 S5 60 6S 70200 I I ••

• ", .• IS :.

•

•

.......; ; _ .

"

.j ._====:=o~

··················~·t············

•r •'.~..~:-=:

500

300

400

­.....~

~e-

•• •

600

132

Figure 4-7. Hole 806B downhole log porosity (solid line) and shipboard

laboratory porosity corrected using combined rebound and

seawater-density correction (dots).

133

80GB Porosity (%)

45200

50 55

e

• e

60

•

65 70

•

•

e

•

c-..,...-.a,..._"!_-o-oo,------o-L-----

•

••

:.

-=---'=-:::.-; : ............................................... . '1" [ 1" ..

i • • ~~~~~~±~~~~::~~i i.·..~······························f······················· <•••••••••••••••••••••••••••~••i":.~~~ ~.·!···············T·················

Ii·: I

500

300

--tI)

,.Q

e 400-.t:: •..c..eu0

600•

134

CHAPTER 5

SUMMARY

This study has addressed several aspects of physical properties (e.g.,

porosity, velocity, acoustic impedance, compression index, expansion index,

and rebound) from the Ontong Java Plateau within the broad framework of

establishing relationships between physical properties and microfossil

content and preservation. The influence of microfossil content on physical

properties is evident.

For example, the study involving actual counts of microfossil

constituents (Chapter 2) indicated relationships between microfossil content

and interparticle porosity, velocity, and impedance. Several factors,

however, lead to problems in determining these relationships, including the

presence of varying amounts of intraparticle porosity and the influence of

incipient cementation on velocity. More accurate estimates of interparticle

porosity are needed, perhaps through analyses of sample SEM images, or a

statistical estimate based on the observed number of intact microfossil tests.

Many of the identified seismic reflectors (Shipboard Scientific Party,

1991a;Mosher et al., 1993) could be related to changes in the relative

percentages of microfossil constituents. In many cases, impedance contrasts

could be related to known paleoceanographic dissolution events.

Examination of data from studies such as Mayer (1993) and Berrera et al.

(1993) may provide additional information regarding the link between

impedance contrasts and paleoceanographic events.

The analysis of consolidation test data (Chapter 3) also indicated a

relationship between physical properties parameters, namely compression

135

index and expansion index, and microfossil content. We found that both

parameters decrease with increasing carbonate content, likely caused by the

incompressibility of carbonate grains relative to the compressibility of

siliceous or clay grains. Compression and expansion indices also decrease

with increasing carbonate content, likely caused by a lower relative volume

of compressible matrix with increasing percentages of relatively

incompressible grains. These results are consistent with those of other

studies for sediments with a range of carbonate contents.

We observed no evidence that microfossil tests were broken and

intraparticle water released during our consolidation testing. This

conclusion is supported by the lack of foraminifer test crushing or breakage

observed in the SEM micrographs, and was also found by Lind (1993) for

consolidation tests, using higher pressures, on ooze samples.

Many studies have used consolidation test data to interpret sediment

stress history. However, the difficulty in determining Pc' for these high

carbonate sediments results in poor estimates of Pc' and, subsequently, of

OCR. The compression index, Cc , is also likely affected. The problem of

determining appropriate Cc and Pc' for sediments with high CaC03 content

(and for other sediments that result in the observed e -log p' curve) bears

further study.

The observed relationship between expansion index and sediment

constituents led to the examination of sediment rebound for correction of

laboratory density and porosity to approximate in situ conditions (Chapter 4).

Rebound-eorrected laboratory data can be used in place of the downhole

logging data when the logging data are missing or erroneous. Many studies

have estimated in situ porosity using Hamilton's (1976) carbonate rebound

136

correction, which is limited in its depth applicability and includes sediments

containing as little as 30% carbonate in the model. Hamilton's correction is

not applicable to Ontong Java Plateau sediments, which generally contain

85% to 95% calcium carbonate and exhibit markedly different rebound

behavior.

Our rebound data from consolidation tests yielded porosity rebounds

of 1% to 4%, significantly lower than those reported for other sediment types,

such as siliceous sediments and clays, which have reported rebounds up to

about 10%. A radiolarian-rich sample exhibits 6% rebound, which is

consistent with the higher porosity rebounds reported for siliceous

sediments, and illustrates the influence of sediment composition on porosity

rebound. Based on the observed variability in rebound for different

sediments, rebound corrections ideally would be performed only when

rebound data is available for a particular sediment type. However, the

rebound correction derived in this study for Ontong Java Plateau sediments

could likely be used for other high-carbonate sediments.

137

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